We report the first evidence of magnetic reconnection driven by advection in a rapidly developing large granule, using high spatial resolution observations of a small surge event (base size 4?? by 4??) with the 1.6 meter aperture New Solar Telescope (NST) at Big Bear Solar Observatory. The observations were carried out in narrow-band (0.5 ?) Helium I 10830 ? and broad-band (10 ?) TiO 7057 ?. Since He I 10830 ? triplet has very high excitation level and is optically thin, its filtergrams enable us to investigate the surge from the photosphere through the chromosphere into the lower corona. Simultaneous space data from Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) were used in the analysis. It is shown that the surge is spatio-temporally associated with magnetic flux emergence in the rapidly developing large granule. During the development of the granule, its advecting flow ( 2 km/ s) squeezed the magnetic flux into an intergranular lane area, where a magnetic flux concentration was formed and the neighboring flux with opposite magnetic polarity was cancelled. During the cancellation, the surge was produced as absorption in He I 10830 ? filtergrams while simultaneous EUV brightening occurred at its base. The observations clearly indicate evidence of finest-scale reconnection process driven by the granule’s motion.
Partially ionized plasma environments, where release of magnetic energy and topological reconfiguration of magnetic fields via magnetic reconnection is known or conjectured to take place, range from highly collisional, e.g. interstellar medium and lower solar chromosphere with ionization fraction below 0.1%, to weakly collisional, e.g. in the upper solar chromosphere with ionization fraction of 1%-10%. Different plasma processes, such as ionization and recombination, ion-neutral interaction via charge-exchange collisions, Hall currents, and radiative losses can become the dominant factors in determining the reconnection rate and the structure of the reconnection region in different parameter regimes. The HiFi multi-fluid modeling framework has been used to implement all of the above processes in a single self-consistent model and to perform 2D simulations of magnetic reconnection under a variety of plasma conditions. Motivated by the simulations, here we present analytically derived predictions of the reconnection rate, as well as the size, shape, and structure of a 2D magnetic reconnection region in a partially ionized plasma under given conditions. The predictions are tested against the simulation results, and implications for better understanding the dynamics of magnetized chromospheric plasmas are discussed.
The solar chromosphere and transition region (TR) form a highly structured and dynamic interface region between the photosphere and the corona. This region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona. Nevertheless, the chromosphere remains poorly understood, because of the complexity of the required observational and analytical tools: the interface region is highly complex with transitions from optically thick to optically thin radiation, from pressure to magnetic field domination, and large density and temperature contrasts on small spatial scales. The Interface Region Imaging Spectrograph (IRIS) was selected for a NASA SMEX mission in 2009 and is scheduled to launch on 26-June-2013 (with first light scheduled for mid July). IRIS addresses critical questions: (1) Which types of non-thermal energy dominate in the chromosphere and beyond? (2) How does the chromosphere regulate mass and energy supply to the corona and heliosphere? (3) How do magnetic flux and matter rise through the lower atmosphere, and what role does flux emergence play in flares and mass ejections? These questions are addressed with a high-resolution near and far UV imaging spectrometer sensitive to emission from plasma at temperatures between 5,000 K and 10 MK. IRIS has a field-of-view of 120 arcsec, a spatial resolution of 0.4 arcsec, and velocity resolution of 0.5 km/s. The IRIS investigation includes a strong numerical modeling component based on advanced radiative MHD codes to facilitate interpretation of observations. We describe the IRIS instrumentation and numerical modeling, and present the plans for observations, calibration and data distribution. We will highlight some of the issues that IRIS observations can help resolve. More information can be found at http://iris.lmsal.com
We present a study using coordinated observations of the Interferometric BIdimensional Spectrometer (IBIS) at the Dunn Solar Telescope and Hinode / Solar Optical Telescope of a quiet sun (QS) region near disk center. Our goal is to analyze the relationship between the cancellation of opposite magnetic polarities in the photosphere and 'Rapid Blueshifted Excursions' (RBEs) by comparing quasi-simultaneous magnetograms and chromospheric H? line profiles. On the one hand, the RBEs are considered the on-disk counterpart of the type-II spicules observed at the limb that are most likely caused by magnetic reconnection on small scales. On the other hand, the magnetic cancellation is a signature after small-scale reconnection in the QS. We developed an automatic tracking algorithm for detecting magnetic cancellation events in the photosphere, based on the existing SWAMIS code which is good at tracking magnetic flux emergence. Our code can find characteristics of each cancellation event and the detected cancellation sites appear to outline the supergranular network. Furthermore, another automatic tracking code for RBEs has been developed for the spectroscopic observations obtained with IBIS. We are able to show a statistical distribution of the properties of RBEs, such as lifetime, shape, and line-of-sight velocity. Finally, using the spatial and temporal tracking of both magnetic cancellation events and RBEs, we find that there is no simple one-to-one correspondence. The majority of RBEs are related to magnetic cancellation events, however a subset of them are not.
Two recent papers report on measuring differential rotation in data that views the Sun as a star. Unlike using tracers at different latitudes to measure the differential rotation, disk-integrated light averages over many latitudes and can only work if the features both exist at a dominate latitude that changes with the solar cycle and they persist long enough to affect the measured rotation rate. Bertello, Pevtsov, and Pietarila (2012, ApJ 761, pg 11) use disk-integrated Ca II K-line data from the SOLIS/ISS instrument to show that a change in rotation rate is clearly visible at the beginning of the current solar cycle in the disk-integrated K-line. Scargle, Keil, and Worden (2013, ApJ in press, arXiv:1303.6303) use the Sacramento Peak K-line series to look at the last current and previous three cycles with fairly strong evidence that the differential rotation is visible in cycle 22, but much harder to see in cycles 21 and 23. In order to understand the differences in the three cycles we report on solar differential rotation measurements in both the Sacramento Peak disk-integrated, Ca II K spectral time series (1977-2012) and full-disk, Ca II K spectroheliogram time series (1977-2002) observed at the Evans Solar Facility. The former data set is the same as used by Scargle et al (2013) and averages about 2-3 measurements per week. For the disk-integrated spectra, we use two interpolation schemes to fill in missing days (regression and singular value decomposition with proxy data sets) and use two methods (power spectra and autocorrelation) to find the rotation rates. We find a clear signature of solar differential rotation for solar cycle 21 and 22 and a partial signature for cycle 23. We test this result by measuring differential rotation using the Ca II K spectroheliograms using phase analysis between longitudinal bands. We have also explored the image features that lead to changes in the disk-integrated spectrum's signal-to-noise. The data analyzed in this presentation can be found at the National Solar Observatory web site http://nsosp.nso.edu/cak_mon/ , or by file transfer protocol at ftp://ftp.nso.edu/idl/cak.parameters and ftp://diglib.nso.edu/Evans_spectroheliograms/.
A quiescent region on the Sun containing three filaments is used to study the properties of mass motion. This study determines if the footpoints or end-points of the filaments are the locations from where mass gets injected into the filaments. Several hypotheses have been put forth in the past to determine how a filament acquires mass. Trapping of coronal mass in the filament channel due to condensation (Martin, 1996) and injection of mass into the filaments during magnetic reconnection (Priest, et al., 1995) are some of the speculations. This study looks for indications for injection of mass via chromospheric footpoints. The data consists of blue (H?-0.5 Å) and red (H?+0.5 Å) wing high resolution H? images of the W29N37 region of the Sun taken on Oct 30, 2010, from 1200 - 1600 UT. The Dutch Open Telescope was used to obtain the data. The images are aligned and animated to see Doppler motion in the fibrils. Smaller fibrils merge to form longer ones; barbs appear and disappear in one of the long filaments and is seen moving along the length of the filament. A region with no typical filament-like absorption feature is observed to be continuously receiving mass. Fibrils appear to be converging from opposite sides along what appears to be a neutral line; mass motion is seen in these fibrils as well. An eruption occurs in a region of fibrils lumped together at the end of the first hour (1300 UT) followed by plage brightening at 1430 UT near one of the filament regions. Helioviewer (Panasenco, et al., 2011) is used for aligning the images; GIMP is used for precision alignment and animation. Each frame in the sequence is studied carefully to note changes in the filament regions. The footpoints of the filaments are determined by the changes observed in the position of the filament ‘legs’ in each frame. Variations in the magnetic polarity corresponding to changes observed in the chromosphere are analyzed using HMI magnetograms. Bright and dark points on the magnetogram surrounding the filaments are examined for possible locations of footpoints. The HMI images are overlaid with Stonyhurst grids and full disk H? images to improve the accuracy in determining the location of the footpoints.
How changes in the three-dimensional magnetic field of solar active region are related to Coronal Mass Ejections (CME) is an important question for contemporary solar physics. Complex active regions are the predominant source of powerful high-speed CMEs, which can result in strong geomagnetic storms. In this paper we present the properties of chromospheric magnetic field of active regions that produced solar flares and CMEs using observations of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility operated by the National Solar Observatory. Currently, the SOLIS Vector Spectromagnetograph (VSM) is the only instrument that is capable of obtaining full Stokes profiles in both the photospheric Fe I ?630.2 nm and chromospheric Ca II ?854.2 nm lines on a daily basis. VSM also has the capability of making rapid scans covering an area sufficiently large to contain an active region. We shall present the Stokes profile characteristics of photospheric and chromospheric lines of few CME source regions.
It is a generally accepted idea from observations that delta-spot regions are harbingers of 'super activity' in the Solar atmosphere. In this computational study we model a typical delta spot region by emerging two independent twisted flux ropes into the Solar corona. We support the above idea by invoking existing statistical studies of delta-spot regions and the associated flares. It has been found from observations that component sunspots locked into a delta-spot region often have different sizes, fluxes and evolutionary history. Also supporting the above idea is the fact that the connectivity between the two component spots is found to be stronger after a flare than before it. The two colliding bipoles can give rise to steep magnetic gradients and shear at the neutral line. Such collisions have the potential to increase the magnetic free energy available for not just one but multiple eruptions. In our isothermal compressible MHD simulations we vary the relative magnetic flux as well as the twist of the two emerging flux ropes. For colliding bipoles we do find signatures of multiple eruptions as well as intense current sheets. Further, we also find evidence of new magnetic connections between the components of the delta-spot after eruption.
We present a statistical survey of almost 10'000 radio type III bursts observed by the Nancay Radioheliograph from 1998 to 2008, covering nearly a full solar cycle. In particular, sources sizes, positions, and fluxes were examined. We find an east–west asymmetry in source positions that could be attributed to a ~6 degrees eastward tilt of the magnetic field, that source FWHM sizes s roughly follow a solar-cycle-averaged distribution dN/ds = 14 ?-3.3s-4 arcmin-1day-1, and that source fluxes closely follow a solar-cycle-averaged dN/dS = 0.34 ?-2.9 S-1.7 sfu-1 day-1 distribution (when ? is in GHz, s in arcminutes, and S in sfu). Fitting a barometric density profile yields a temperature of 0.6 MK, while a solar wind-like (~h-2) density profile yields a density of 1.2 × 106 cm-3 at an altitude of 1 Rs, assuming harmonic emission. Finally, the flux distribution combined with rough radiative efficiency estimates hint at the possibility that escaping electron beams might carry as much energy away from the corona as is introduced into it by nanoflare-accelerated electrons.
The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument is a two-channel imaging spectrograph that observes the solar corona and transition region with high spectral resolution and a rapid cadence made possible by unprecedented sensitivity. The 2013 flight on 23 April at 17:30 UT incorporated a new wavelength channel covering the range 525–630 Å, the previously-flown 300-370 Å channel, and the first flight demonstration of cooled active pixel sensor (APS) arrays, resulting in high-signal-to-noise spectral coverage spanning a wide temperature range of 0.025 to 10 MK. Absolute radiometric calibration of the two channels is performed using a hollow cathode discharge lamp and NIST-calibrated AXUV-100G photodiode. For the 2013 flight, EUNIS co-observed dynamic coronal phenomena with DST/IBIS, SoHO/CDS, SDO/AIA and Hinode/EIS and contributes to the absolute radiometric calibrations of these instruments. Plans for future wavelength channels to cover the AIA 94 and 131 Å bandpasses and address the currently unresolved spectral lines (and therefore temperature responses) within them are presented.
We analyze the meridional cross-section of the 3D coronal electron density in the range from 1.5 to 4 $R_\odot$ obtained by the tomography method during minimum and beginning of maximum of solar activity corresponding to February 2008 and July 2011, respectively. The importance of this coronal region is that it contains the transition from closed to open magnetic coronal structures. At the moment, only STEREO/COR1 provides observation that provides information on the coronal structure in this region. Therefore, analysis of 3D coronal density structure is critical for deriving the position where transition from closed to open magnetic coronal structures occurs. The 3D coronal density in the region of interest has been obtained by applying tomographic techniques to white light coronagraph data obtained by STEREO/COR1 instrument. It is shown that enhanced density structures associated with coronal streamers and pseudostreamers have a tendency to become radially directed at heliocentric distances of about 3 and 2 $R_\odot$ during minimum and maximum of solar activity, respectively. Potential Field models (PFSS) with several different values of the Source Surface position have been analyzed for consistency with the obtained 3D coronal density structure.
Various models for ion heating in the corona predict ion temperatures that are anisotropic with respect to the magnetic field. Such anisotropy is observed in-situ in the solar wind, but has not been measured in the solar corona. Here, we present a method for performing such measurements and apply the technique to an observation of an equatorial coronal hole. For the analysis we combined spectroscopic line width measurements with a magnetic potential field source surface model. We then studied how the line width varied as a function of the inclination angle between the line of sight and the magnetic field direction. Perpendicular to the magnetic field, the ion temperatures and non-thermal velocities agree with those inferred from off-limb measurements, as expected. For the parallel component, we find that the temperature is uniform for all the ion species. The parallel temperature is expected to reflect the proton temperature, which we inferred to be about 1.8 +/- 0.2 MK. Using a similar technique, but applied to Doppler shifts, we found the outflow velocity to be about 5 km/s in the coronal hole.
Recently, line width measurements using EUV Imaging Spectrometer data have verified previous suggestions that line widths decrease with height over polar coronal holes. This implies that Alfven waves are damped at lower heights than predicted by simple models. However, in order to quantify the amount of energy dissipated, it is necessary to separate the thermal and non-thermal contributions to the line width. We present an analysis which determines the ion temperature and non-thermal velocity at the lowest heights (< 1.1 R_sun) by using the observation that at those heights the waves are not damped and by assuming that in that region the ion temperature is constant. We then extrapolate these results to larger heights (> 1.1 R_sun), where damping is observed, and estimate the energy dissipated from the waves and length and time scales for the damping. The analysis implies that the dissipated energy matches that required to heat the coronal hole and accelerate the fast solar wind and that the waves are dissipated over a length of about 0.2 R_sun or a time of about 70 s.
We present a narrative of the launch and early evolution of a flux rope comprised within the coronal mass ejection (CME) that left the Sun on 12 December 2008. The two STEREO spacecraft were near quadrature at that time, so we were afforded a unique view of this flux rope from along its edge and down its barrel simultaneously using STEREO's EUVI cameras. We find that a sequence of seemingly separate CMEs observed in the corona and solar wind were actually manifestations of the same flux rope passing through the imagers' fields of view at different times. The launch begins with a small solar flare at the northern-most end of a pre-formed flux rope, which lifts off from this end first via the tether-cutting mechanism. Other segments of the flux rope follow this launch, and a filament is observed to roll over the top of these segments and pour back into the solar disk, thereby indicating the mass draining mechanism at play. The southern end of the flux rope remains fixed to the Sun, leading to an eventual stress-fracture and bisection of the flux rope. The severed southern end eventually disconnects from the Sun a day later via what appears to be the kink instability mechanism. This narrative, describing the interplay between three separate onset mechanisms for simple CME during a period of extremely low solar activity, demonstrates the complexity of the physics of CME onset.
Coronal Mass Ejections (CMEs) are one of the most important solar phenomena in affecting conditions on Earth. There is not a consensus as to the physical mechanisms responsible for ejecting CME material from the solar atmosphere. Measurements that specify basic physical properties close to the Sun, when the CME is still evolving, should be useful in determining the correct theoretical model. One of the best observational techniques is that of Faraday rotation, a rotation in the plane of polarization of radio waves when propagating through a magnetized medium like the corona. The importance of Faraday rotation in determining the structure and evolutionary history of CMEs was discussed in Liu et al (ApJ 665, 1439, 2007). In this paper, we report Faraday rotation observations of ``constellations'' of background extragalactic radio sources near the Sun on three days in August, 2012, with the intention of observing a source occulted by a CME. Observations were made with the Jansky Very Large Array (VLA) of the National Radio Astronomy Observatory. We made polarization measurements at 6 frequencies between 1.31 and 1.94 GHz. On August 2, 2012, a CME clearly visible on the LASCO C3 coronagraph occulted a radio source from our sample, 0843+1547. Preliminary data analysis shows a Faraday rotation transient for 0843+1547 which appears to be associated with the CME. The Faraday rotation measure changes from nearly 0 before CME passage, to a value of about -12 radians/square-meter before declining after CME passage. We will discuss the interpretation of these data in terms of models for CME structure, as well as the status of our observations of other sources on August 2, and on other days. This work was supported at the University of Iowa by grant ATM09-56901.
We investigate the characteristics of coronal heating using a systematic technique that analyzes the properties of nanoflares in active regions (AR). Our technique computes cooling times, or time lags, using SDO/AIA light curves of all of the coronal AR emission, including the so-called diffuse emission. We recently presented results using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk 2012). We found that the majority of the pixels had cooling plasma along their line of sight, consistent with impulsive coronal nanoflare heating. Additionally, our results using the AIA 94 channel data showed that the nanoflare energy is stronger in the AR core and weaker in the AR periphery. Are these results representative of the nanoflare characteristics exhibited in the majority of active regions, or is AR 11082 unique? Here we present the time-lag results for a survey of active regions and determine whether these nanoflare patterns are born out in other active regions as well. This research was supported by the NASA Heliophysics Guest Investigator program.
We observe an eruptive jet that occurred in an on-disk solar coronal hole, using EUV images from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA), supplemented by magnetic data from the SDO Helioseismic and Magnetic Imager (HMI). This jet is similar to features variously called macrospicules or erupting minifilaments. After an initial pre-eruptive phase, a concentration of absorbing, cool material in the AIA images moves with a substantially-horizontal motion toward a region of open magnetic field, and subsequently jets out along that vertical field. Prior to and during the jet's ~20 min lifetime, the magnetic flux integrated over the local region shows flux changes of <~20% of the background flux levels, with a time-averaged emergence rate of no more than <3 × 10^15 Mx/s in the neighborhood of the jet. Contrary to some jet models, there was no substantial recently-emerged bipolar field in the base of the jet. Instead, there was an established evolving magnetic arcade that held mini-filament-like cool plasma in its core field. We propose that subtle evolution of the magnetic flux in and around this arcade destabilized its core field, as in some standard-sized arcade blowout eruptions that produce a flare and CME following the slow rise of a standard-sized filament in the core of the arcade. Closed field carrying the cool plasma erupted into the open field and formed the blowout jet, evidently at least partly by interchange reconnection with the open field. Internal reconnection made compact bright 'flare' loops inside the blowing-out arcade, while, on the outside, interchange reconnection made longer and dimmer EUV 'crinkle' loops. That the loops made by the external reconnection were considerably larger than the loops made by the internal reconnection makes this event a new variety of blowout jet, a variety not recognized in previous observations and models of blowout jets.
Fe XIV 530.3 nm emission features typically appear at high latitudes near solar maximum and gradually migrate towards the equator, merging with the sunspot 'butterfly diagram' (cf. Altrock, 1997, Solar Phys. 170, 411). In 1999, persistent Fe XIV coronal emission appeared near 70° in the north and began migrating towards the equator at a rate 40% slower than the previous two solar cycles. Discerning this process in the south is difficult. However, In 2009 and 2010 an acceleration occurred in both hemispheres. In Cycles 21 - 23 solar maximum occurred when the the maximum number of Fe XIV emission regions per day (averaged over 365 days and both hemispheres) first reached latitudes of 20° ± 1.7°, at similar times in the north and south. Currently the greatest number of emission regions is at 21° in the north and 24° in the south. This indicates that solar maximum is occurring now in the north but not yet in the south. Another typical high-latitude process is the 'Rush to the Poles' of polar crown prominences and their associated coronal emission, including Fe XIV. The Rush is a harbinger of solar maximum (cf. Altrock, 2003, Solar Phys. 216, 343). Cycle 24 displays an intermittent Rush that is only well-defined in the northern hemisphere. In 2009 an initial slope of 3.6 °/yr was found in the north, compared to an average of 9.4 ± 1.7 °/yr in the previous three cycles. However, in 2010, following a discontinuous jump of 9°, the slope increased to 5.6 °/yr. Solar maximum in Cycles 21 - 23 occurred when the center line of the Rush to the Poles reached 76° ± 2°, again at similar times in the north and south. In the northern hemisphere this already occurred at 2012.0 ± 0.3. In the southern hemisphere the Rush is very poorly defined. A linear fit to several maxima will reach 76° in the south at 2014.2, implying delay in solar maximum in the south until then. Compared to this is evidence that sunspot areas and numbers in the northern hemisphere reached a maximum in late 2011 and in the southern hemisphere in 2012. The official smoothed sunspot number evidently reached a maximum in early 2012.
Coronal hole jets have been suggested to make a significant contribution to the solar wind, but spectroscopic measurements - which provide the best means of measuring the mass flux - have been rare due to the difficulty of capturing jets with narrow slit spectrometers. A continuous two day coronal hole measurement made with the Hinode/EIS instrument is reported, which has revealed over 30 jets seen in the coronal Fe XII 195.12 (1.5 MK) emission line. More than half of the events have no clear signature in SDO/AIA or Hinode/XRT images and thus represent a separate class of jet events not previously reported. The jets show a wide range of morphologies, ranging from classic columnar features to mini-CME events. Statistics of the events will be presented: their relation to bright points and the coronal hole boundary; durations; and repeated occurrence. Velocity, density and temperature measurements derived from the EIS spectra will be presented, and mass flux estimated. The photospheric signatures of the jets, as determined from SDO/HMI, will be shown in a separate presentation (Muglach & Young).
Jets in EUV and X-ray images have been observed for several decades. Many of them originate in coronal holes which have received special interest as possible contributors to the solar wind. A new class of CH jets are observed with Hinode/EIS which show pronounced signatures in EUV spectral lines. Coronal velocities of up to 200 km/s and enhancements in line width are measured. On the other hand these jets are hardly visible in imaging data like SDO/AIA and Hinode/XRT (see contribution by Young and Muglach). In this contribution we will investigate the photospheric signatures at the footpoint of these jets. We use SDO/HMI magnetograms to show the evolution of the magnetic flux. From SDO white light images we derive the plane-of-sky flow field using local correlation tracking. Both the evolution of the magnetic field and the flows allow us to study the photospheric drivers of these jets.
Magnetic fields in the corona can be approximated by using PFSS (Potential Field Source Surface) model in conjunction with magnetogram measurements of the photosphere. This approach is incorporated here to find locations of magnetic null points in the solar corona. Observations from WSO (Wilcox Solar Observatory) provide the necessary harmonic coefficients for a PFSS model. We located all magnetic null points in the PFSS model going back to Carrington Rotation 2098. The time and location where they cross the West limb is compared to high resolution observations made by SDO/AIA. Variations in predicted and observed null point locations, and estimates of the duration of each null, will be examined. This work will provide a catalog of coronal nulls observed by SDO that can be examined further for interesting dynamical behavior or variations in neighboring plasma.
Although the mechanisms responsible for heating the Sun's corona and accelerating the solar wind are still actively investigated, it is largely accepted that photospheric motions provide the energy source and that the magnetic field must play a key role in the process. Verdini et al. (2010) presented a model for heating and accelerating the solar wind based on the turbulent dissipation of Alfven waves. We first use our time-dependent model of the solar wind to reproduce Verdini et al's solution; then we extend its application to the case when the energy equation includes thermal conduction and radiation losses, and the upper chromosphere is part of the computational domain.Application of this formulation to our 3D MHD model of the solar corona and solar wind will be discussed.
We study the partial eruption of a solar filament observed by the SDO and the STEREO-A spacecraft on 9 May 2012. This filament was located in active region NOAA 11475, and consisted of two distinct branches, separated in height above the active region's primary polarity-inversion line. For two days prior to the filament eruption, several threads of filament material were observed to connect the lower branch to the upper branch with evidence of a transfer of mass along them. The eruption commenced as a slow rise of the upper branch that began at 9 May 2012 23:40 UT, with the main eruption occurring half an hour later, producing a coronal mass ejection (CME). During the eruption, the upper branch was observed to rotate approximately 120 degrees in a counter-clockwise direction. We suggest that the mass transfer events also comprised a transfer of magnetic flux that led the upper branch of the filament to lose equilibrium as a result of a helical kink instability or torus instability.
All theories that attempt to explain the heating of high temperature plasma observed in the solar corona are based on short bursts of energy. The intensities and velocities measured in the cores of quiescent active regions, however, can be steady over many hours of observation. One heating scenario that has been proposed is the “long nanoflare storm,” where short duration heating events occur infrequently on many sub-resolution strands; the intensity of the strands are then averaged together to explain the observed steady structures. We examine the intensities predicted by nanoflare storms by modeling an arcade of strands in an active region core. We explore the term “infrequently” in these nanoflare storms by modeling several storms with various heating rates and magnitudes until the heating is nearly steady. Comparisons of the predicted intensity ratios (IFeXII/IFeXV and ICaXVII/ICaXIV) for Hinode’s EUV imaging spectrometer (EIS) with recent observations indicate that a long nanoflare storm with a specific heating rate and magnitude can match some of the observed intensity ratios while steady heating predicts EIS intensity ratios that are too low to match observations. Furthermore, we can quantify when the heating becomes steady as the intensity ratios become nearly constant when the heating rate is greater than 6 heating events per 1000 seconds.
We present a comparison of the solar corona observed during the total solar eclipses on 2010 July 11 and on 2012 November 13. The white light images were taken at Easter Island in 2010 and at Northeast Queensland, Australia, in 2012; while the concurrent EUV images were take with SDO/AIA and PROBA2/SWAP. The 2010 eclipse was observed at the beginning of Sunspot Cycle 24 , which peaked near our 2012 observation. We compare a plethora of corona features in the white light images and reveal some interesting differences in the enhanced EUV images taken by SDO/AIA and PROBA2/SWAP. We construct potential field models using our newly refined Coronal Modeling System (CMS2) software with line-of-sight photospheric magnetograms from SDO/HMI. The source surface heights derived from detailed comparison between our models and observations are compared to the standard source-surface model. We also compare the dynamics of the two eclipse observations. Similar to the 2010 eclipse, a CME was observed using temporally spaced eclipse images. We address unresolved problems in the models and observations with the hope of correcting them for future eclipse observations, such as the 2017 total solar eclipse across the continental U.S. References  Pasachoff, J. M., Rusin, V., Druckmüllerová, H., Saniga, M., Lu, M., Malamut, C., Seaton, D. B., Golub, L., Engell, A. J., Hill, S. W., Lucas, R., 2011, ApJ, 734, 114
We investigated the internal structure of a coronal loop that occurs naturally in our 3D simulation based on an Alfven turbulence heating model. The thermal structure above the active region is in a thermal non-equilibrium state. As the system evolves, it develops a tunnel-like low-temperature region along a small bundle of field lines. The EUV emissivity of this structure also exhibits a coronal loop along this temperature tunnel, which has a nearly uniform cross section. Over the course of about 27 hours solar time, multiple loops occur at discrete locations. Some of them carry high-speed plasma flows. The structure and dynamics of these loops will be analyzed.
Magnetic reconnection between open and closed magnetic field in the corona is believed to play a crucial role in the corona / heliosphere coupling. At large scale, the exchange of open /closed connectivity is expected to occur in pseudo-streamer structures. However, there is neither clear observational evidence of how such coupling occurs in pseudo-streamers, nor evidence for how the magnetic reconnection evolves. Using a newly-developed technique, we enhance the off-limb magnetic fine structures observed with AIA and identify a pseudo-streamer-like feature located close to the northern coronal hole. We first identify that the magnetic topology associated with the observation is a pseudo-streamer, null-point-related topology bounded by open field. We then compare the evolution of the observed pseudo- streamer fine structure in the location of strong currents, i.e. in the region of energy dissipation, with the dynamics of the magnetic field resulting from the interchange reconnection obtained in a fully 3D MHD simulation. The morphological and dynamical similarities between the pseudo-streamer observations and the results from the simulation strongly suggest that the evolution of the pseudo-streamer is caused by interchange reconnection in a null-point topology that is embedded in Quasi-Separatrix layers. Besides identifying the mechanism at work in the large-scale coupling between open and closed field, our results highlight that interchange reconnection in pseudo-streamers is a gradual physical process that differs from the impulsive reconnection of the solar-jet model.
We characterize the physical properties of two coronal streamers during Earth/Ulysses quadrature configurations for the previous two solar minimum periods. Comparisons between coronal remote-sensing observations and in situ measurements of solar wind plasma properties are being used to characterize the origin of slow wind streams. In order to investigate slow solar wind heating and acceleration, we compare the measurements with predictions from MHD models. We aim to use the empirical measurements to distinguish between different proposed physical processes for the slow solar wind. This work is supported by NASA grant NNX10AQ58G to the Smithsonian Astrophysical Observatory.
We re-investigate the 2.5-dimensional Chen & Shibata (2000) emerging flux model of flux rope destabilization and coronal mass ejection initiation in the context of a stratified atmosphere. Using the high-fidelity numerical simulation framework HiFi, we evaluate the model's sensitivity to free parameters and boundary conditions. By way of multiple numerical experiments, which are much more feasible in 2D than in 3D, we explore the parameter space to address the stability of the initial magnetic equilibrium and the subsequent magnetic reconnection following its disruption due to flux emergence. In particular, we study the behavior of the initial X-point below the flux rope and investigate the factors that determine the direction of the X-point collapse into a current sheet, resulting in either the flux rope's eruption into the corona or its descent into the chromosphere.
Solar filaments exhibit a range of eruptive-like dynamic activity from the full, or partial, eruption of the filament mass and surrounding magnetic structure, as a CME, to a fully confined dynamic evolution or ‘failed’ eruption. On 2011 June 7, a dramatic partial eruption of a filament was observed by multiple instruments and SDO and STEREO. One of the interesting aspects of this partial eruption was the response of the surface as non-erupting material fell back under the influence of gravity. The impact sites show clear evidence of brightening in the observed EUV wavelengths due to energy release by the impact. There are two plausible physical mechanisms of the brightening: heating of the plasma due to the kinetic energy of the impacting material - compression of the plasma, or reconnection between the magnetic field of the low-laying loops with the field carried by the impacting material, or combination thereof. By analyzing the emission of the brightenings in several SDO/AIA wavelength, and comparing the kinetic energy of the impacting material (with true velocity determined from triangulation of the two STEREO spacecraft) to the radiative energy we provide clues for the dominant mechanism of energy release involved in the observed brightenings.
Sigmoids are sinuous structures located in active regions that have characteristic “s-shaped” or inverted “s-shaped” loops. Active regions containing sigmoids are observed to have higher rates of flaring and CMEs. Previous work detailing the properties of sigmoids has generally focused on specific case studies of a handful of regions. Although such studies are representative of the structure and evolution of these regions, significant insight can be gained by an observational overview approach with systematic and statistical analysis of a large sample of sigmoids. We present a new sample of 72 sigmoidal regions observed in a wide wavelength range and in different parts of the solar atmosphere by various instruments such as the Hinode/XRT, SDO/AIA, STEREO, and LASCO. From this data we compiled a comprehensive list of many different parameters including: size and aspect ratio, presence of Ha or EUV filaments, flare and CME association, number of sunspots, active region and sigmoid lifetimes, etc. Our preliminary results show that sigmoids have a higher eruption rate than other active regions. We also find that the ratio of the long axis to short axis of the sigmoids has a strong peak at 2.5 and the lifetime peaks at 2 days. We also follow the evolution of the magnetic flux in the photosphere and derive whether the sigmoids appear during the emergence or cancellation stages of active region evolution. These results can provide constraints for models of flux rope evolution in global simulations.
Recent observations of sungrazing comets have opened an exciting novel methof of probing the solar atmosphere. As well as providing valuable insight on the magnetic field of the lower corona, sungrazing comets also promise the potential of measuring the solar wind as their detritus follows the open field lines of the corona. In this work, we model the chemisty of the material sublimated from comets as they skim across the Sun. This material, largely water ice, is rapidly dissociated and ionized by the solar radiation field and coronal electrons. We track the evolution of the ionizing material as it expands into the corona using a generalized Haser-like model. Based on these results, we have predicted the emission resulting from these ions in different regions of the corona and compared the results with SDO/AIA observations.
We use the Horizontal Current – Current Sheet (HCCS) magnetic field model to characterize the coronal hole/streamer boundaries in the corona for the Solar Cycle 23 Minimum (1996-1998). The HCCS model describes the interface region much more accurately than traditional PFSS (potential field – source surface) models. Once the models are computed we can compute expansion factors and magnetic field strengths anywhere in a 3D grid. These data are combined with coronal outflow velocities and electron densities from UVCS and LASCO on SOHO to show how the coronal plasma parameters (at 2.3 solar radii) relate to the magnetic field geometry.
Comet ISON is a large sun-grazing comet due to pass perihelion on November 28, 2013. It will go through the corona 2.7 Rsun above the surface, much higher than earlier EUV comets. We will use our time-dependent models of cometary debris to discuss how the trail of Comet ISON can be used to probe the solar corona. The debris trail left behind as a sun-grazing comet passes by the Sun undergoes different chemical processes at different distances from the Sun. Near the Sun the material is rapidly converted to atomic ions and becomes part of the solar corona. Far from the Sun the evaporated material can remain in molecular form for a long time, while the grains of asteroidal material can exist long enough to become meteors in planetary atmospheres. The larger fragments may survive as independent comets, until the next perihelion passage. In between those limits the material moves in the solar wind acceleration region. The debris could become entrained in the solar wind and be measured by satellites far from the Sun. This material would be observed as abundance anomalies in the solar wind. We will describe the fate of the cometary debris trail left by Comet ISON and what the trail can tell us about the solar corona and solar wind.
We present an updated analysis of a RHESSI observation of flare-accelerated electrons in the core of a coronal mass ejection (CME) and examine their role in heating the CME. Previous CME observations have found remarkably high temperatures in the core. A joint observation by RHESSI and AIA of a partly occulted flare on 2010 November 3 allows us to test the hypothesis that this excess energy is collisionally deposited by flare-accelerated electrons. Extreme ultraviolet (EUV) images from AIA show an ejected plasmoid forming the CME core and sheath, with isothermal multifilter analysis revealing temperatures of >~11 MK in the core. RHESSI images produced via the Two-Step CLEAN algorithm reveal a large (~100 x 50 arcsec2), diffuse hard X-ray source matching the location, shape, and evolution of the AIA plasma, indicating that the emerging CME core is filled with energetic electrons. Spectroscopy demonstrates that the nonthermal electrons contain enough energy to heat the CME. The time integral of the EUV emission matches the hard X-ray light curve (similar to the ``Neupert effect'' observed in soft and hard X-ray time profiles), directly linking the CME temperature increase with the nonthermal electron energy loss. This is the most direct observation to date of energetic flare electrons heating a CME, emphasizing the close relationship of the two in solar eruptive events.
Quasi-periodic propagating intensity disturbances (PDs) have been observed in cool (about 1 MK) coronal loops in EUV images over a decade. They are widely accepted to be slow magnetosonic waves since their propagation velocity is close to the coronal sound speed. However, recent spectroscopic observations from Hinode/EIS revealed their association with persistent coronal upflows, making this interpretation debatable. Motivated by the scenario that the observed persistent upflows could be cumulative result of numerous individual flow pulses generated by sporadic heating events (nanoflares) at the loop base, we constructed a broadband velocity driver with repetative tiny pulses, whose energy frequency distribution follows the flare power-law scaling distribution. We then performed 3D MHD modeling of an idealized bipolar active region by applying this broadband velocity driver at the footpoints of coronal loops which appear open in the computational domain. Our model successfully reproduced the propagating disturbances with similar features as the observed. We find, based on our simulations, that upflow pulses unavoidably excites a slow magnetosonic wave fronts propagating along the loop with the phase speed which is much larger than the local flow speed as the flow velocity decreases with height. Our modeling results support that the observed PDs are mainly the signature of waves above the footpoints of the loops, and suggest that the observed PDs and associated persistent upflows may be driven by the same mechanism such as impulsive heating at the loop base.
The Solar Dynamics Observatory (SDO) Feature Finding Team (FFT) module; the 'Spatial Possibilistic Clustering Algorithm' (SPoCA), provides solar active region (AR) and coronal hole (CH) data to the Heliophysics Event Knowledgebase (HEK) every four hours. The HEK was queried for CH and AR data catalogs from 2010 to 2012 that have been used to calculate coronal synodic rotation rates and investigate latitudinal movement of non-polar CH and AR. The rotation rate calculated for all ARs within the time range was found to be ? = 12.96(±0.030) - 2.12(±1.70) sin2(?) deg/day which is close to earlier studies. The CH rotation rates determined varied (?0.5 deg/day) depending on the latitude ranges considered, for CHs within 50 degrees of the equator the calculated rotation rate is ? = 13.81(±0.033) - 3.46(±0.501) sin2(?) deg/day. The first components from the rotation rates for CHs are similar to recent studies using a different time period, but the second component found in this study indicates that the there is a differential profile and not completely rigid rotation. This supports the idea that differential profile may change over the course of the solar cycle. Some preliminary analysis of trends in CH and AR areas and their respective rotation rates is also discussed.
We report on Doppler shifts observed in HeII ?304 with the Multi-Order Solar EUV Spectrograph (MOSES) sounding rocket. We use wavelet analysis for automated identification and analysis of Doppler shifted emission at different spatial scales, and track the temporal evolution over time of flight.
Prominence cavities appear as circularly shaped voids in coronal emission over polarity inversion lines where a prominence channel is straddling the solar limb. The presence of chromospheric material suspended at coronal altitudes is a common but not necessary feature within these cavities. These voids are observed to change shape as a prominence feature rotates around the limb. We apply temperature diagnostics to SDO data to investigate the thermal structure. We find significant evidence that the prominence cavity is hotter than the corona immediately outside the cavity boundary. This investigation follows upon ``Thermal Properties of A Solar Coronal Cavity Observed with the X-ray Telescope on Hinode'' by Reeves et al., 2012, ApJ, in press. M. Weber and K.K. Reeves are supported under contract NNM07AB07C from NASA to SAO. T. Kucera is supported by an award from the NASA SHP Program.
The discovery of 'EIT waves' rekindled interests in what used to be called flare waves, which had been typically observed in H-alpha. In addition to Moreton waves, first observed at the Lockheed Solar Observatory, other manifestations of shock waves propagating in the corona include type II radio bursts and filament oscillations away from flare sites. Identification of EIT waves with the postulated fast-mode MHD shock waves in the corona has been questioned, however, largely because of their low speeds (e.g., 200-400 km/s). EIT's 10-20 minute cadence could be a contributing factor for this, and we need to find how fast large-scale coronal propagating fronts are in higher-cadence EUV images. It is clear that AIA on SDO is the best instrument at the moment for this type of work. With the availability of high-cadence full-disk images, we now can compare propagating fronts in different directions, and determine the highest speed of each event on AIA images more objectively and accurately than on EIT (and STEREO EUVI) images. In a large number of EIT wave events, we have measured speeds of propagating fronts using AIA's 193 A images. Before the fronts are deflected by the discontinuities, e.g., active regions and coronal holes, the mean and median speeds are 620 km/s and 600 km/s, respectively, and many exceed 800 km/s. Higher speeds are often seen in events that accompany a type II burst, strong flare or energetic CME, but the distribution of the speed with these attributes is broad. We also find that the speeds of the large-scale coronal propagating fronts are not well correlated with those of the associated CMEs. Given that large-scale coronal propagating fronts at large distances represent freely propagating MHD waves, we discuss how to understand their nature close to their origins.
Solar prominences observed at high resolution consist of long, narrow threads of cool plasma seen in emission above the limb (and in absorption against the disk in filaments). It is generally accepted that this structure outlines the prominence magnetic field, which mechanically supports the cool mass against gravity and thermally insulates it against conduction from the surrounding hot corona. The origin of the narrow widths of prominence threads is not yet well understood. We are investigating the idea that the width is set by a competition between the gravitational instability of a dense fluid (the prominence) residing above a dilute fluid (the corona) and the stabilizing influence of magnetic tension forces when the prominence field is distorted. A key issue is the effect on the Rayleigh-Taylor instability of a sheared magnetic field whose direction rotates continuously through the body of the prominence. Linear analysis indicates that marginal stability is reached for aspect ratios (parallel to perpendicular wavelengths) of about 25:1 for solar parameters, with unstable modes having still larger ratios. We describe high-resolution numerical simulations of initially monolithic slab prominences in which the prominence/corona interface fragments due to the growth of shear-modified Rayleigh-Taylor instabilities from an initial spectrum of small-amplitude velocity fluctuations. In the nonlinear regime, the cold, dense prominence plasma aggregates at cross-field scales on the order of 200 km, limited by the grid resolution, whereas the thread lengths are about 100 Mm. A comparison simulation with an unsheared prominence field forms threads that descend readily to low altitudes due to reduced magnetic support against gravity, while another simulation with no field (plasma only) rapidly disintegrates isotropically into very small bubbles and spikes. This work was supported by NASA’s LWS TR&T program.
It has recently been proposed that prominences play an important role as return flows of the chromosphere-corona mass cycle, in which hot plasma is transported upward in forms of spicules and prominence bubbles (likely due to flux emergence), while cool plasma drains downward in forms of vertical prominence threads (Berger et al. 2011 Nature). A critical step in this cycle is the condensation of the million-degree coronal plasma into T<10,000 K prominence material by a radiative cooling instability (i.e., thermal non-equilibrium), as numerically simulated (Karpen & Antiochos 2008; Xia et al. 2012) and first evidenced in recent SDO/AIA observations (Liu et al. 2012; Berger et al. 2012 ApJL). Such a runaway cooling process occurs in coronal loops of various sizes and generally leads to condensation at magnetic dips and formation of funnel-shaped prominences. A moderate-sized prominence can drain a significant mass of typically 10^15 gram/day, which is comparable to the mass of a CME or a fraction of the entire corona. Here we present a survey of funnel prominences that appear to be common in AIA observations at various locations and times. We find longer cooling times in longer/taller coronal loops whose densities are lower, consistent with the expected quadratic dependence on density of the optically-thin radiative loss. We propose that such funnel prominences, usually small in size, can constitute a new type of prominences, and similar processes can produce elementary building blocks of large-scale quiescent prominences in filament channels. This picture is supported by the recent theoretical development on spontaneous formation of current sheets and condensations manifested as prominence threads (Low et al. 2012a, b, ApJ).
Solar polar jets are dynamic, narrow, radially extended structures observed in EUV emission. They have been found to originate within the open magnetic field of coronal holes in “anemone” regions, which are generally accepted to be intrusions of opposite polarity. The associated embedded-dipole topology consists of a spine line emanating from a null point atop a dome-shaped fan surface. Previous work (Pariat et al. 2009, 2010) has validated the idea that magnetic free energy stored on twisted closed field lines within the fan surface can be released explosively by the onset of fast reconnection between the highly stressed closed field inside the null and the unstressed open field outside (Antiochos 1996). The simulations showed that a dense jet comprising a nonlinear, torsional Alfven wave is ejected into the outer corona on the newly reconnected open field lines. While proving the principle of the basic model, those simulations neglected the important effects of gravity, the solar wind, and an expanding spherical geometry. We introduce those additional physical processes in new simulations of reconnection-driven jets, to determine whether the model remains robust in the resulting more realistic setting, and to begin establishing the signatures of the jets in the inner heliosphere for comparison with observations. Initial results demonstrate explosive energy release and a jet in the low corona very much like that in the earlier Cartesian, gravity-free, static-atmosphere runs. We report our analysis of the results, their comparison with previous work, and their implications for observations. This work was supported by NASA’s LWS TR&T program.
Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the 'Sun-as-a-point source' mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indices (total solar magnetic and Poynting fluxes) during several months of data, when multiple active regions are present on the solar disk. We discuss capabilities of the two approaches, and what we can learn from them.
The solar corona is filled with loop-like structures that appear bright against the background when observed in the extreme ultraviolet (EUV). These loops have several remarkable properties. Warm loops (? 1 MK) appear to be ? 2 ? 9 times as dense at their apex as predicted by of hydrostatic atmosphere models. These loops also appear to be of constant cross-section despite the fact that the field strength in a potential magnetic field should decrease in the corona, causing the loops to expand. Why many active region loops appear to be of constant cross-section is not well understood. Theories range from an internal twist of the magnetic field to observational effects. In this work we simulate active region loops with different expansion factors heated by nanoflare storms. We calculate the hydrodynamic properties for each loop as a function of the expansion factor Gamma. We show that even modest tapering ratios can lead to drastic changes in the density profiles of active region loops, and they can also explain the overpressure at the apex of these loops. Synthetic AIA images of each loop are made to show the observable consequences of the expansion of loops near the instrumental resolution. We find that all loops, even those with a large expansion factor, appear to be of near constant cross-section when images are simulated in AIA passbands. Only when the images are simulated for a much higher resolution instrument with 0.1” pixels does the real expansion of the loop become apparent.
We present results from 3D numerical MHD simulations, which show how the partial emergence of twisted magnetic flux tubes from the convection zone, and their interaction with background coronal magnetic fields, leads to the formation of unstable magnetic configurations in the corona. These unstable configurations are capable of initiating the ejection of a flux rope. Our studies improve upon the traditional approach of driving the magnetically-dominated corona with kinematic boundary conditions by explicitly including the dynamic emergence of flux through the convection zone and lower, pressure-dominated, solar atmosphere. By showing that magnetic flux emergence is capable of initiating coronal ejections, we can root these dynamic events in the convection zone and hence to the source of solar activity, the solar dynamo, which is a vital step in improving our understanding of space weather.
Coronal cavities are circular darkened regions observed above the solar limb in white light and EUV coronal images. It is a region of low density relative to the surrounding corona. In this study, we are using synoptic maps made from EUV images from the Atmospheric Imager Assembly (AIA) instrument and vector magnetogram images from Helioseismic and Magnetic Imager (HMI) on the SDO to determine the structure and evolution of cavities. The EUV synoptic maps, constructed from circular rings above the limb, are found to best show cavities in 211Å (Fe XIV, ~2.0 MK) and 193 Å (Fe XII, ~1.6 MK) and171 Å (Fe IX, ~0.6 MK) pass bands. Moreover, 304Å (He II, ~0.05 MK) synoptic map best shows the evolution of prominence associated with cavity. Magnetogram synoptic map constructed from the central meridian, shows the underlying magnetic structure of the cavity and prominence. We have also used EUV synoptic map to construct the polar view of the cavities. DEM analysis was used to calculate the temperature and density of the cavities. The high spatial and time resolution combined with the broad temperature coverage provides a consistent picture of the cavity material and the dynamics of the structure.
While there is accumulated evidence of high temperature coronal emission in active region cores that corresponds to structures in equilibrium, other studies have found of evolving loops. We investigate the EUV intensity variations of two low and short coronal loops observed in the core of NOAA AR 11250 on 13 July 2011 between UT 12:02 and 16:32. The loops (32 Mm loop 1, 23 Mm loop 2), run directly between the AR opposite polarities, and are first detectable in the 94Å band (effective temperature ~ 7 MK). Space-time slices present intermittent brightenings evocative of turbulence. Spatial averages over the intermoss loop region lead to light curves used to analyze the temporal evolution of the loops. We find quantities with scaling regimes that are characteristic of intermittent processes. In particular intensity histograms display scaling ranges with slopes ~ -1.8, and spectra also show a scaling region for frequencies 1-8 mHz, with slopes – 3.8 (loop 1) and -2.8 (loop 2). We further investigate the time evolution of the loops in five other AIA EUV channels. The results are separated into two classes. Group A (94Å, 335Å, 211Å) characterized by hotter temperatures (~2-6 MK), and group B (193Å, 171Å, 131Å) by cooler temperatures (0.4 – 1.6 MK). In loop 1 (group A) the intensity peaks in the 94Å channel are followed by maxima in the 335 Å channel with a time lag of ~10 min, suggestive of a cooling pattern with an exponential decay. The 211Å maxima follow those in the 335 Å channel, but there is no systematic relation which would indicate a progressive cooling process. In group B the signals in the 171 and 131Å channels track each other closely, and tend to lag behind the 193Å. The three signals follow a general gradual increase reaching a maximum at about the middle of the time series and then decrease. An exponential cooling model can also be associated with the 193 and 171Å pair. For loop 2 the observations in the group B light curves present similar properties as in loop 1. In contrast the intensity curves in group A only show one distinct case which could be a candidate for exponential decay via a 94 Å to 335 Å cooling process.
The FORWARD suite of SolarSoft IDL codes converts an analytic or simulation data cube into a form directly comparable to observations. Observables such as extreme ultraviolet, soft X-ray, white light, and polarization images from the Coronal Multichannel Polarimeter (CoMP) can be reproduced. The observer's viewpoint is also incorperated in the forward analysis and the codes can output the results in a variety of forms in order to easily create movies, Carrington maps, or simply plasma properties at a particular point in the plane of the sky. We present a newly developed front end to the FORWARD codes which utilizes IDL widgets. Our ultimate goal is to provide as useful a tool as possible for a broad range of scientific applications.
Quasi-periodic fast-mode magnetosonic wave trains both inside and outside expanding CME bubbles have recently been discovered by SDO/AIA (Liu et al. 2011, 2012; Shen & Liu 2012). In general, a wave train inside a CME bubble originates from a flare site and propagates along a funnel of coronal loops at typically 1000-2000 km/s (Ofman et al. 2011). A wave train outside a CME usually originates from a CME flank and propagates in the low corona along the solar surface following the leading front of a global EUV wave at typically 500-1000 km/s. The former is primarily seen in the cooler 171 Angstrom channel with a characteristic temperature of 0.8 MK, while the latter is pronounced in the hotter 193 and 211 Angstrom channels of typically 1.6-2.0 MK. What is the relationship between the two types of wave trains? Why do they appear differently in location and wavelength (temperature)? To answer these questions, we report here for the first time the evidence that the wave train beyond the CME bubble is the continuation of the same wave train along the funnel within the CME. The continuous deceleration of the waves is consistent with the expected decrease of the local fast-mode speed with distance from the active region (e.g., Ofman et al. 2011; Downs et al. 2012). There is an abrupt change of the wave speed at the topological interface where the expanding CME flank is located, indicative of contrasting magnetic and plasma conditions, which can give rise to different (fast-mode) speeds and wavelength (temperature) dependent appearances of these wave trains.
A total eclipse swept across Queensland and other sites in northeastern Australia on the early morning of 14 November 2012, local time. We mounted equipment to observe coronal images and spectra during the approximately 2 minutes of totality, the former for comparison with spacecraft images and to fill in the doughnut of imaging not well covered with space coronagraphs. Matching weather statistics, viewing was spotty, and our best observations were from a last-minute inland site on the Tablelands, with some observations from a helicopter at 9000 feet altitude over our original viewing site at Miallo. Only glimpses of the corona were visible at our Port Douglas and Trinity Beach, Cairns, locations, with totality obscured from our sites at Newell and Miallo, though some holes in the clouds provided coronal views from Palm Cove and elsewhere along the coast. Preliminary analysis of the spectra again shows Fe XIV stronger than Fe X, as in 2010 but not earlier, a sign of solar maximum, as was the coronal shape. An intriguing CME is discernible in the SE. Acknowledgments: We thank Terry Cuttle, Aram Friedman, Michael Kentrianakis, and Nicholas Weber for assistance and collaboration in Australia and Wendy Carlos for image processing. Our expedition was supported in part by NSF grant AGS-1047726 from Solar Terrestrial Research of the Atmospheric and Geospace Sciences Division, and by the Rob Spring Fund and Science Center funds at Williams College. ML was also supported in part by a Grant-In-Aid of Research from the National Academy of Sciences, administered by Sigma Xi, The Scientific Research Society (Grant ID: G20120315159311). VR and MS acknowledge support from projects VEGA 2/0003/13 and NGS-3139-12 of the National Geographic Society. We are grateful to K. Shiota (Japan) for kindly providing us with some of his 2012 eclipse coronal images.
Tether-cutting reconnection has been widely recognized as an important mechanism for producing solar flares. Using 12 minutes cadence vector magnetograms obtained with SDO/HMI and flare UV images taken by SDO/AIA, we study the 2011 February 13 M6.6 flare concentrating on the magnetic field structure immediately before and after the event. We analyze two groups of specific field lines, which are extrapolated under the nonlinear force-free field (NLFFF) assumption and stem from the regions of four conspicuous flare UV kernels at the event onset. It is found that the connectivity of these field lines evolves dramatically from the preflare to the postflare state mimicking the occurence of a tether-cutting reconnection. We use the magnetic twist derived from the NLFFF to further examine the evolution of the field structure.
A solar flare is composed of impulsive energy release events by magnetic reconnection, which forms and heats flare loops. Recent studies have revealed a two-phase evolution pattern of UV 1600A emission at the feet of these loops: a rapid pulse lasting for a few seconds to a few minutes, followed by a gradual decay on timescales of a few tens of minutes. Multiple band EUV observations by AIA further reveal very similar signatures. These two phases represent different but related signatures of an impulsive energy release in the corona. The rapid pulse is an immediate response of the lower atmosphere to an intense thermal conduction flux resulting from the sudden heating of the corona to high temperatures (we rule out energetic particles due to a lack of significant hard X-ray emission). The gradual phase is associated with the cooling of hot plasma that has been evaporated into the corona. The observed footpoint emission is again powered by thermal conduction (and enthalpy), but now during a period when approximate steady state conditions are established in the loop. UV and EUV light curves of individual pixels may therefore be separated into contributions from two distinct physical mechanisms to shed light on the nature of energy transport in a flare. We demonstrate this technique using coordinated, spatially resolved observations of UV and EUV emission from the footpoints of a C3.2 thermal flare.
He I D3 line has a unique response to the flare impact on the low solar atmosphere and can be a powerful diagnostic tool for energy transport processes. Using high-resolution and high-cadence images obtained from the recently digitized films of Big Bear Solar Observatory, we report D3 observation of the M6.3 flare on 1984 May 22, which occurred in an active region with a circular magnetic polarity inversion line (PIL). The impulsive phase of the flare starts with a main elongated source that darkens in D3, inside of which bright emission kernels appear at the time of the initial small peak in hard X-rays (HXRs). These flare cores subsequently evolve into a sharp emission strand lying within the dark halo simultaneously with the main peak in HXRs, reversing the overall source contrast from -5% to 5%. The radiated energy in D3 during the main peak is estimated to be about 10^30 ergs, which is comparable to that carried by nonthermal electrons above 20 keV. Afterwards the flare proceeds along the circular PIL in the counterclockwise direction to form a dark circular ribbon in D3, which apparently mirrors the bright ribbons in Halpha and He I 10830 A. All these ribbons last for over one hour in the late gradual phase. We suggest that the present event resembles the so-called black-light flare that is proposed based on continuum images, and that D3 darkening and brightening features herein may be due to, respectively, the thermal conduction heating and the direct precipitation of high-energy electrons.
Solar flares accelerate electrons up to hundreds of MeV and heat plasma to tens of MK. In large (GOES M- and X-class) flares, in addition to the 10-25 MK plasma thought to be the result of chromospheric evaporation, even hotter plasma (up to 50 MK) may be directly heated in the corona. While observations of hard X-ray bremmstrahlung directly probe the nonthermal electron population, for large flares the spectra below 20-30 keV are typically dominated by thermal emission. The low energy extent of the nonthermal spectrum can be only loosely quantified by hard X-ray spectrometers, resulting in significant implications for calculating flare energy budgets and for constraining possible acceleration mechanisms. A precise characterization of the thermal emission is imperative. Extreme ultraviolet observations from the EUV Variability Experiment (EVE) on-board the Solar Dynamics Observatory (SDO), combined with X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI), currently offer the most comprehensive view of the flare temperature distribution. EVE observes EUV emission lines with peak formation temperatures of 2-20 MK, while RHESSI observes the X-ray bremsstrahlung of hot, 10-50 MK plasma; combined, the two instruments cover the full range of flare plasma temperatures. In this work, we handle the EVE-RHESSI data for a few large flares in three steps; first we calculate differential emission measures (DEMs) using EVE and RHESSI independently for purposes of cross-calibration. Second, we create combined EVE-RHESSI DEMs, fixing the nonthermal spectral parameters to those found using a RHESSI-only spectral fit. The final step is to unconstrain the nonthermal parameters (in particular, the low-energy cutoff of the spectrum) and let them be fit in the same process as the EVE-RHESSI DEM, to obtain a fully self-consistent thermal plus nonthermal model. This research is supported by NASA Heliophysics Guest Investigator Grant NNX12AH48G.
The angular variation of high energy electrons during a solar flare is key to understanding the acceleration mechanism. Regularised inversion of RHESSI X-ray spectra, using the effect of photospheric albedo, allows us to estimate the angular distributions of the emitting electrons. The results for all flares studied are consistent with an isotropic pitch-angle distribution, and inconsistent with a ratio of downward to upward going electron flux greater than 3:1. To attempt to understand these results, I have performed stochastic simulations of electron pitch-angle scattering by Coulomb collisions, including the effects of collisional energy loss, and of magnetic field convergence. This allows us to estimate what constraints these observations put on the parameters of the electron beam, such as initial directionality, and of the characteristics of the loop itself. These simulations suggest that Coulomb collisions cannot sufficiently isotropise the distribution to be consistent with the observations, even for an initially isotropic injected distribution.
The February 15, 2011 solar flare was the first X-class flare of Solar Cycle 24, and as such has received much attention in the literature. This flare has many interesting features, including the presence of a sun-quake, a large coronal mass ejection, and a distinct “pre-impulsive” phase during which the thermal emission rises appreciably before any significant rise in non-thermal flux. It is presently unknown why certain flares exhibit pre-impulsive behavior, or what heating or particle acceleration mechanisms are responsible for these events. In addition, there has been limited analysis of the hard x-ray emission from this particular flare. We present x-ray images, spectra, and analysis of RHESSI data from this event with particular focus on the pre-impulsive phase. Additional SDO AIA and HMI images will be used to determine the context of the RHESSI observations. Looking at the early stages of this and similar flares presents excellent opportunities to constrain acceleration and heating modes and to learn more about what physical processes underlie flare and CME initiation.
Solar energy storage and release events are interconnected. This research addresses one aspect of the interconnection of those energetic solar events. Addressed aspect emerged in recent research that showed a movement of a flux tubes generates oscillations. Analysis was performed using six C class and one M class flares during Dec. 25, 2011. Using AIA and HMI data, we investigated the connection between flare induced disturbance and changes in the flux of photospheric oscillations. Results showed significant increase of oscillatory flux following the flare itself. This increase was detected outside of flare location. However, we noted that in the small area of the open field there is no significant response. Results show a need for deeper statistical analysis of the oscillatory response at flare induced disturbance. This kind of analysis might reveal energy distributions when this aspect of interconnection is in question.
The collisional thick-target model has been used to explain many spectral features of solar flares. Flare classification, based on soft X-rays observed by GOES, should then depend on the model. Using a combination of numerical simulations and observed features of flares, we explore the sensitivity of flare classification to the parameters of the thick-target model. We vary the total non-thermal energy, spectral index, and the cut-off energy of the electron beam one at a time for two sets of parameters derived from flares observed with RHESSI. We find that the classification depends strongly on non-thermal energy, only weakly on spectral index, and that the cut-off energy can either increase or decrease the GOES class, depending upon how it varies.
According to current understanding, solar flares occur when magnetic reconnection releases magnetic energy stored in the corona. Current sheets are essential elements in models of fast magnetic reconnection which demand large electric fields on small scales. While current sheets are also associated with magnetic energy storage, they are not the actual site at which energy is stored: free magnetic energy is stored throughout the coronal volume. This means that reconnection on very small scales must initiate the release of energy stored on much larger scales. Some insight into this cross-scale coupling can be gained from simplified, semi-analytic models of transient reconnection in a finite-length current sheet. In one such model, presented here, the localized reconnection electric field launches a fast magnetosonic pulse carrying the sheet's current at its front. Magnetic energy is converted in place, by the pulse, into bulk kinetic energy of reconnection inflow and outflow. The model predicts, for example, the fraction of stored energy directly thermalized, or converted to other forms such as magnetosonic waves and bulk flows. This work was supported by a joint NSF/DOE grant.
The solar corona is well known for its highly structured appearance. This structuring is partly due to its magnetic field, and partly due to the complex distribution of mass within the field. Coronal mass density is set by coronal heating which might be constant (the steady-heating picture) or might be sporadic (the so-called nanoflare picture). In the latter scenario, a mass flux occurs through a process referred to as chromospheric evaporation. Reconnection and subsequent loop contraction generate shocks in the corona which result in thermal conduction fronts. These fronts impulsively deposit heat into the cooler chromosphere and drive supersonic upward flows which is the evaporation. This process has been extensively studied in the past, but generally using models with uniform magnetic field connecting the corona and chromosphere. Transonic flows are known, in general, to be highly sensitive to variation in the cross-section though which they are driven. It is therefore expected that the complex structure of the magnetic canopy could have a dramatic effect on the supply of mass into the corona. We explore this possibility using a simplified 1-D hydrodynamic model of evaporation occurring through a varying magnetic field canopy.
The collisional thick-target model has been used to explain many spectral features of solar flares. Flare classification, based on soft X-rays observed by GOES, should then depend on the model. Using a combination of numerical simulations and observed features of flares, we explore the sensitivity of flare classification to the parameters of the thick-target model. We vary the total non-thermal energy, spectral index, and the cut-off energy of the electron beam one at a time for two sets of parameters derived from flares observed with RHESSI. We find that the classification depends strongly on non-thermal energy, only weakly on spectral index, and that the cut-off energy can either increase or decrease the GOES class, depending upon how it varies.
In this study, we present the imaging spectroscopy of an X-class flare with white-light emission on September 06, 2011, observed with Helioseismic Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO). The HMI provides seeing-free images at 6173 \AA continuum with a 45s cadence and six-point spectrograms centered at 6173.34 \AA with 0.172 \AA steps. Taking advantage of the 0.5\arcsecond image scale, the flare kernels are fully resolved and fine structures, including the core and halo, are able to be identified. We analyzed the line-profile, constructed from six spectral positions, of the flare core and halo pixels, respectively. We studied the morphology of the continuum flare kernel comparing with previous white-light observations. The resemblance and the discrepancy of the two kinds of spectra, which could be related to different heating mechanisms, are then discussed.
The EUV Variability Experiment (EVE) onboard SDO is now making routine observations of the solar EUV irradiance on flare timescales. While these observations are important for determining the Sun's influence on the geospace environment, they also provide a crucial diagnostic of the flaring chromosphere during the impulsive phase. Milligan et al. (2012) presented new observations from EVE which showed unambiguous, spectrally and temporally-resolved detections of enhanced free-bound (and free-free) continua during the first X-class flare of Solar Cycle 24; an X2.2 flare that occurred on 15 February 2011. By constructing lightcurves of the Lyman continuum of H, the He I and He II continua, and the Lyman-alpha and He II 304A lines, the authors showed that they peaked in concert with the 25-100 keV emission observed by RHESSI, indicating a chromospheric origin. Allred et al. (2005; RADYN) predicted that these recombination continua should dominate energetically over line emission. By fitting the RHESSI HXR spectra for the same event with a thick-target component, we were able to model the EUV emission observed by EVE in response to the measured beam parameters using RADYN. Here we present the similarities and differences between the observed and modelled emission.
Data from the Multiple EUV Grating Spectrograph (MEGS-A) component of the Extreme Ultraviolet Experiment (EVE) onboard the Solar Dynamics Observatory (SDO) were used to quantify the contribution of continuum emission to each of the EUV channels of the Atmospheric Imaging Assembly (AIA), also on SDO, during an X-class solar flare that occurred on 2011 February 15. Both the pre-flare-subtracted EVE spectra and the exponential fits to the free-free continuum from Milligan et al. (2012) were convolved with the AIA response functions of the seven EUV filters over a 75-minute period at 10 s cadence. The 94A, 131A, 193A, and 335A channels each showed a 15-35% contribution from continuum emission throughout the main phase of the flare. No measurements could be made for the 171A channel which exhibited strong coronal dimming during the event. The continuum contribution to the 304A channel was negligible due to the presence of the strong He II line, while up to 75% of the emission in the 211A channel was found to originate from continuum processes. These findings suggest that the contribution of free-free continuum emission due to thermal bremsstrahlung during flares is more significant in AIA observations than that stated in previous studies which used synthetic from CHIANTI, rather than observed spectra. These results highlight the importance of spectroscopic observations carried out in conjunction with those from imaging instruments so that the data are interpreted accurately.
Strong heating of the chromosphere and transition region during flares results in ribbons containing plasma at temperatures ranging from a few thousand K to 10 million K. We have used SDO to construct emission measure maps in the extended rise phase of the M1.0 event SOL2010-08-07T17:55 using the method of Hannah & Kontar (2012), allowing a pixel-by-pixel examination of the development of thermal plasma in the ribbons, and detailed comparison with the ribbons' magnetic environment. Using RHESSI hard X-ray observations we set limits on the non-thermal emission from the ribbons, and examine the contribution of energy loss by non-thermal electrons to the ribbon heating in this phase.
We report on a campaign at the Dunn Solar Telescope which resulted in successful imaging and spectroscopic observations of a C1.1 solar flare on 18th August 2011. This flare exhibited ribbons with complicated fine structure at the resolution of the DST/IBIS instrument, and a number of bright kernels with sizes comparable to the smallest scales sampled by IBIS, around 2-4 pixels (0.'3-0.'6) FWHM. We focus on these bright kernels, describing their spatial characteristics in the core and wing of H alpha and Ca II 8542, and in the UV and EUV with SDO. We also show preliminary broad-band spectroscopy of the kernels which may demonstrate the presence of an optical continuum in this small flare.
We present a simplified analytic model of a quadrupolar magnetic field and flux rope to model coronal mass ejections. The model magnetic field is two-dimensional, force-free and has current only on the axis of the flux rope and within two currents sheets. It is a generalization of previous models containing a single current sheet anchored to a bipolar flux distribution. Our new model can undergo quasi-static evolution due either to changes at the boundary or to magnetic reconnection at either current sheet. We find that all three kinds of evolution can lead to a catastrophe known as loss of equilibrium. Some equilibria can be driven to catastrophic instability either through reconnection at the lower current sheet, known as tether cutting, or through reconnection at the upper current sheet, known as breakout. Other equilibria can be destabilized through only one and not the other. Still others undergo no instability, but evolve increasingly rapidly in response to slow steady driving (ideal or reconnective). One key feature of every case is a response to reconnection different from that found in simpler systems. In our two-current sheet model a reconnection electric field in one current sheet causes the current in that sheet to increase rather than decrease. This suggests the possibility for the microscopic reconnection mechanism to run away.
It is well known that large solar flares do not occur in isolation. Once an active region produces a flare, the probability of another flares increase dramatically. Using data from NASA's Solar Dynamics Observatory (SDO) and the US Air Force's Improved Solar Observing Optical Network (ISOON), we are able to examine all layers of the solar atmosphere and isolate active regions as they transit the solar disk. By studying the fluctuations in lightcurves and magnetic flux before, during, and after solar flares, we seek to understand the patterns of activity during phases of the active region evolution. In this study, we examined the month of February 2011. During that time, there were fifteen numbered NOAA Active Regions, which produced over 100 flares greater than C1.0, including the first X-class flare of Solar Cycle 24.
We present models of the solar atmospheric response to heating from a beam of flare-accelerated ions. The ions heat the atmosphere through Coulomb collisions which we model by solving a Fokker-Planck kinetic equation. This method models how relativistic ions propagate through the solar atmosphere and includes the effects of pitch-angle scattering through Coulomb collisions, synchrotron emission and magnetic mirroring in a multi-species, partially-ionized plasma. We have performed simulations for a wide variety of injected ion beam energy spectra including those predicted from stochastic acceleration models. The atmospheric response is modeling by solving the radiative hydrodynamic equations in 1D using the RADYN code. This code solves the radiative transfer equation for non-LTE, non-equilibrium optically-thick transitions which dominate in the chromosphere allowing direct comparisons with observed transition profiles. We compare our predicted values for plasma velocities, emission measures, and temperature structure with observations of solar flares from the RHESSI, SDO, and Hinode observatories.
In two-ribbon flares, the fact that the ribbons separate in time is considered evidence of magnetic reconnection. However, in addition to the ribbons separating, they can also elongate (as seen in animations of, for example, the Bastille Day flare). The elongation is undoubtedly related to the reconnection spreading in the out-of-plane direction. Indeed, naturally occurring magnetic reconnection generally begins in a spatially localized region and spreads in the direction perpendicular to the reconnection plane as time progresses. For example, it was suggested that X-line spreading is necessary to explain the observation of X-lines extending more than 390 Earth radii (Phan et al., Nature, 404, 848, 2006), and has been seen in reconnection experiments. A sizeable out-of-plane (guide) magnetic field is present at flare sites and in the solar wind. Here, we study the effect of dissipation mechanism and the strength of the guide field has on X-line spreading. We present results from three-dimensional numerical simulations of magnetic reconnection, comparing spreading with the Hall term to spreading with anomalous resistivity. Applications to solar flares and magnetic reconnection in the solar wind will be discussed.
The differential emission measure of solar flare plasmas was constructed using observations from the EUV Variability Experiment (EVE) and the Markov-Chain Monte Carlo method. Emission lines from ions formed over the temperature range Log T = 5.8 - 7.4 allow for the evolution of the DEM to be studied over a wide temperature range at 10s cadence. The DEM construction technique is applied to several M and X-class flares where impulsive phase EUV emission is observable in the disk-integrated EVE spectra. The emission is verified using AIA images to be originating from the flare ribbons and footpoints and EVE observations are used to infer the thermal structure of the EUV emitting flare chromosphere. For the nine events studied the constructed differential emission measures have a two component distribution during the impulsive phase. The low temperature component has peak temperatures of 1 - 2 MK, and a high temperature component peaking at 10 MK.
We study the relationship between the field changes in the photosphere and the reconnection processes in the corona by comparing the locations of abrupt permanent changes of the magnetic field during strong flares observed by the GONG and HMI instruments and hard X-ray (HXR) emission observed by RHESSI. The chromospheric HXR emission in solar flares is generally believed to mark the footprints of magnetic field lines newly reconnected in the corona. Also, the footpoint motions away from the neutral line are considered to be indicative of the reconnection occurring in arcade magnetic fields at different heights. Our analysis of six flares that occurred during the declining phase of cycle 23 shows that the strongest field changes are well correlated in space, with the HXR footprints moving away from the neutral line in later stages of the flare. The majority of field changes and HXR footpoints are spatio-temporally related but not simultaneous. We will also compare changes in the HMI vector magnetic field measurements with the location and evolution of flare footpoints and discuss possible implications for topology of the magnetic field at the reconnection site in the corona.
We analyze RHESSI hard X-ray spectrum data from the 19 January 2005 GOES X1.3 flare to determine between two models which is a better description of the observed spectrum. The two spectral models we consider are the return current, and a single power law beam with a sharp low-energy cutoff. We show that the reduced chi-squared values arising from fitting each model to the observed spectrum are too close to enable us to decide which model is a better description. However, other methods are available. We demonstrate the application of techniques from Bayesian data analysis that allow us to quantify the relative probability of flare models, and show for the data studied, the return current model is preferred.
In two of the best-observed flares of the last cycle, the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) satellite found the centroids of ion and relativistic electron emission to have a significant displacement. This result is surprising; co-spatially accelerated ions and electrons are thought to be transported along the same field lines, implying they would enter the chromosphere together and have similar emission locations. The Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS) balloon instrument will investigate particle transport in solar flares by providing enhanced imaging, spectroscopy and polarimetry of gamma/HXR flare emission (20keV - 10MeV). GRIPS’ key technological improvements over the solar state of the art in HXR/gamma ray energies (RHESSI) include three-dimensional position-sensitive germanium detectors (3D-GeDs) and a single-grid modulating collimator, the multi-pitch rotating modulator (MPRM). The 3D-GeDs allow GRIPS to Compton track energy deposition within the crystal. This capability (1) enables the MPRM design by acting as a second modulation grid, (2) provides significant background rejection and (3) makes solar polarization measurements possible. The MPRM imager provides quasi-continuous resolution from 12.5 – 162 arcsecs with 2x the throughput of a dual grid collimator system like RHESSI. This spatial resolution can resolve the separate footpoints of many flare sizes. In comparison, RHESSI images with a minimum of 35 arcsecs for gamma-rays, making these footpoints resolvable in only the largest flares. Here, we present the intial calibration of GRIPS’ 3D-GED detectors using laboratory radioactive sources. We evaluate charge sharing between adjacent strips, the detection of coincidences and preliminary depth measurements. The detectors have been shown to have a linear response and resolve line emission. The MPRM modulation grid is constructed and we present initial results from calibration. GRIPS is scheduled for a test flight from Fort Sumner , and two Antarctic long-duration balloon flights (LDBFs).
Set to fly in the Fall of 2013 from Ft. Sumner, NM, the High Energy Replicated Optics to Explore the Sun (HEROES) mission is a collaboration between NASA Marshall Space Flight Center and Goddard Space Flight Center to upgrade an existing payload to make unique scientific measurements of the Sun (during the day) and astrophysical targets (at night) during a single flight. HEROES will use grazing-incidence x-ray focusing optics combined with position-sensitive detectors to make new high energy (~20 keV to 75 keV) observations of the Sun in order to understand particle acceleration in solar flares. The HEROES science payload consists of 8 mirror modules, housing 109 grazing incidence replicated optics, mounted on a carbon-fiber-Aluminum optical bench 6 m from a matching array of focal-plane detectors (high pressure xenon gas scintillation proportional counters). HEROES will investigate electron acceleration and transport in the solar corona both in the solar flares and in the non-flaring quiet Sun. HEROES will image the Sun with an angular resolution of 20 arcsec (FWHM) and will have a sensitivity up to ~50 times better than RHESSI at 20 keV. During 6 hours of solar observations (a minimum requirement for a typical balloon flight), HEROES has a ~75% chance of observing at least one flare with a GOES class above C1, and a ~20% chance of at least one flare above M1. HEROES is expected to observe the faint HXR emission from electrons streaming down the legs of magnetic loops or escaping along open magnetic field lines. Experience on this flight will be used to design of new balloon payload (Super HERO) capable of capable of observing the Sun for 2-4 weeks using a Long Duration Balloon (LDB). This mission is funded by the NASA HOPE (Hands On Project Experience) Training Opportunity awarded by the NASA Academy of Program/Project and Engineering Leadership in partnership with NASA's Science Mission Directorate, Office of the Chief Engineer, and Office of the Chief Technologist.
Strong evidence exists that coronal loops as observed in EUV and soft X-rays are not monolithic isotropic structures, but are more accurately modeled as bundles of independent strands. Modeling the observed active region transient brightenings (ARTBs) within this framework allows exploration of the energetic ramifications and characteristics of these stratified structures. Here we present a simple method of detecting and modeling ARTBs observed with the Hinode X-Ray Telescope (XRT) as groups of simple 0-dimensional strands. We probe parameter space to understand better the spatial and temporal dependence of strand heating in impulsively heated loops. We present results from using this partially automated method to analyze observations and gain a statistical insight into the heating of these structures.
Polars are cataclysmic variables, close binaries consisting of a strongly magnetic white dwarf (WD) and a cool solar-like secondary star. They tend to have high and low photometric states, corresponding to times of high accretion (due to Roche lobe overflow from the secondary) and low accretion (where the accretion source is under debate). Since tidal spin-up forces the secondary to rotate at the orbital period, typically < 1 day, it should be very magnetically active. We use a solar flux rope/CME model with field strength Bsec, placed in a strong external field (BWD), to explore the stability of magnetic loops on the secondary in the presence of the megagauss WD field. We find that for low ratios BWD/Bsec, loops confining a prominence separated by more than a certain distance are stable, but as the ratio is increased, a second regime of instability for widely separated loops appears. The two instability regimes grow with BWD/Bsec, until above a certain value, no loops are stable. We find that for reasonable masses of loop confined material, the BWD induced instability may be able to explain much of the accretion seen in polar low states, as well as several other observed properties. Implications for polar evolution are discussed. This research was supported by several NSF grants.
Relativistic antiparticles can be created in high-energy nuclear interactions; thus, detection of antiparticles can tell us something remarkable about the underlying high-energy processes and nuclear interactions. However, once created, the antiparticles remain a minor fraction of their conjugant normal particles, so the detection of the antiparticles represents a big science challenge. To address this challenge we employ imaging and polarimetry of microwave radiation produced as the positrons gyrate in the ambient magnetic field. The key property of the radiation used in this method is that the oppositely charged particles, electrons and positrons, produce radiation with opposite helicity, easily distinguishable by currently operating and future radio facilities. Analysis of available spatially resolved microwave data augmented by independent magnetic field measurements allows us to remotely detect the relativistic positron component in several solar flares; here we present one of them as a vivid example. We expect that coming years will bring an opportunity to routinely detect the relativistic positron contributions in flares using new radio facilities, including Jansky VLA and ALMA, having an ability to image circular polarization at the relevant high microwave/mm frequencies. This will provide us with invaluable information on the relativistic positron spectra, spatial distribution, and evolution in solar flares, and help to much better constrain the nuclear component of the flare-accelerated particles.
The Second NWRA Workshop to compare methods of solar flare forecasting was held 2-4 April 2013 in Boulder, CO. Many researchers who are active in the field participated, and provided their methods' prediction results on standardized datasets. We discuss what is necessary to make meaningful comparisons of methods, focusing on techniques for removing bias and estimating random errors, and present preliminary method comparisons based on standardized skill scores. Funding for the workshop and the data analysis was provided by NASA/LWS contract NNH09CE72C and NASA/GI contract NNH12CG10C.
The Second NWRA Workshop to compare methods of solar flare forecasting was held 2-4 April 2013 in Boulder, CO. This is a follow-on to the First NWRA Workshop on Flare Forecasting Comparison, also known as the ``All-Clear Forecasting Workshop'', held in 2009 jointly with NASA/SRAG and NOAA/SWPC. For this most recent workshop, many researchers who are active in the field participated, and diverse methods were represented in terms of both the characterization of the Sun and the statistical approaches used to create a forecast. A standard dataset was created for this investigation, using data from the Solar Dynamics Observatory/ Helioseismic and Magnetic Imager (SDO/HMI) vector magnetic field HARP series. For each HARP on each day, 6 hours of data were used, allowing for nominal time-series analysis to be included in the forecasts. We present here a summary of the forecasting methods that participated and the standardized dataset that was used. Funding for the workshop and the data analysis was provided by NASA/Living with a Star contract NNH09CE72C and NASA/Guest Investigator contract NNH12CG10C.
It can be argued that the most stringent test of understanding a deterministic system is to be able to forecast an outcome based on observable particulars. It can also be argued that (1) solar flares may not be deterministic , and even if they were, our present understanding is nowhere close to being able to predict the time and location of a solar flare with any certainty. Still, solar flare prediction is a needed component of our national space weather infrastructure, and many groups around the world are investigating ways to improve forecasting methods, especially in light of new observational data available, such as from the Solar Dynamics Observatory. We present a (very) brief report of the 'state of the field', summarizing insights gained from workshops (held in 2009 and 2013) aimed at head-to-head comparisons of flare forecasting methods in specific contexts. In summary, today's methods combine sophisticated data analysis with statistical or computer-learning algorithms generally result in probabilistic forecasts. It is unclear whether any of the presently developed methods clearly outperforms the others, as measured using standard skill scores applied to the careful comparisons that participating researchers engaged in at the workshops. However, it is also clear that new insights into flare triggering mechanisms, especially as afforded by modern analysis of high-cadence, high-quality data such as from SDO, have yet to be fully exploited. Funding for the workshops and subsequent analysis was provided by NASA/Living with a Star contract NNH09CE72C and NASA/Guest Investigator contract NNH12CG10C.
Coronal mass ejections (CMEs) from the polar crown filament region originate above 60-degree latitude during solar activity maxima. Cessation of these high-latitude CMEs marks the end of the maximum phase when the solar poles reverse their polarity. The eruption mechanism of these polar CMEs is not well understood: they originate from bipolar magnetic regions in contrast to the low-latitude ones, which may occur from both bipolar and multipolar regions. One of the key questions is whether the polar CMEs are associated with flare-like brightening and if so what the nature of the CME-flare relationship is. We investigated a number of polar CMEs, which do have post-eruption arcades (PEAs) observed in soft X-ray and EUV wavelengths. We combine data from the Solar Dynamics Observatory (SDO), the Solar Terrestrial Relations Observatory (STEREO), and the Solar Heliospheric Observatory (SOHO) to examine the flare-CME relationship in the case of polar crown CMEs. In particular, we measure the initial CME acceleration and compare it with the time-derivative of the PEA intensity to demonstrate a relationship akin to Neupert effect. The CME morphology, mass, and kinematics are similar to the low-latitude CMEs. The ratio of the thermal energy content of PEAs to the CME kinetic energy is very similar to that from low-latitude CMEs. Finally, the free energy estimated from photospheric magnetograms and the area covered by the PEAs, we find that the free energy is sufficient to power the CMEs and PEAs. Thus, we conclude that the polar-crown CMEs are fundamentally similar to the low-latitude CMEs and hence may have similar eruption mechanism.
The relationship between X-ray and UV emission during flares, particularly in the context of quasi- periodic pulsations, remains unclear. To address this, we study the X-ray and UV emission during the eruptive flare of 2011 June 7 utilising X-ray imaging from RHESSI and UV 1700A imaging from SDO/AIA. This event is associated with synchronous quasi-periodic pulsations in both the X-ray and UV emission. Remarkable behaviour of the hard X-ray footpoints is also observed during the course of this event. The footpoint associated with the left-hand flare ribbon is shown to reverse direction along the flare ribbons on at least two occasions. Over the same time interval, the footpoints also gradually move apart at v ? 12 km/s, implying a height increase of the reconnection site. This is consistent with the measured plane-of-sky thermal X-ray source velocity of ? 14 km/s, and matches the gradual outward expansion of the UV ribbons. However, there is no associated short-timescale motion of the UV bright regions. We find that the locations of the brightest X-ray and UV regions are offset, particularly during the early portion of the flare impulsive phase. Correlation analysis of the measured flare arcade parameters reveals that the 25 - 50 keV hard X-ray flux is only weakly correlated with the other arcade properties, despite begin strongly correlated with the integrated UV emission. We characterise this event in terms of long-term behaviour, where the X-ray nonthermal, thermal, and UV emission sources appear consistent, and short-term behaviour, where the emission sources are inconsistent. We suggest that the short timescale behaviour of hard X-ray footpoints, and the nature of the observed quasi-periodic pulsations, is determined by fundamental, as-yet unobserved properties of the reconnection region and particle acceleration sites. This presents a challenge for current three-dimensional flare reconnection models.
Flare quasi-periodic pulsations (QPPs) have been observed over a vast energy spectrum, from radio to hard x-rays. The periodicities of these fine structures range from tens of milliseconds to tens of seconds and suggest highly structured but intermittent energy release. In some cases, the sources of microwaves and thermal hard x-rays are situated near the apex of the flare loop arcades and are not stationary. Although it is unclear whether all the observed varieties of QPPs can be explained via a single, unified process, our recent high-resolution simulations of a breakout eruptive flare (Karpen et al. 2012) indicate that spatially and temporally localized reconnection is a plausible candidate for these bursts of radiation. With our null-tracking capabilities, we follow the creation and evolution of X- and O-type nulls in the flare current sheet and characterize their periodicity. QPPs located at the apex of the flare arcade may result from the interaction of downward-moving islands in the sheet with the arcade below. Each island is composed of highly twisted magnetic field lines that comprise a single reconnected flux tube. Upon arrival at the top of the flare loops, secondary reconnection events between the island and the arcade produce discrete energy release events that could be related to observed QPPs in that region. Different regimes of current-sheet reconnection (slow/fast), island sizes, rates of island coalescence, and rates of reconnection between islands and arcades may all help to explain the variety of energy and time scales exhibited by the flare QPPs.
White-light continuum and hard X-ray emission in flares have strong correlations in time, but at present we do not have a clear idea about their height structures. Recently, several studies of the relative positions of the white-light and hard X-ray sources have been made using observations of flares near the solar limb. However, these results are still inconclusive due to the small number of flares observed. On 28 January 2011 a white-light flare (SOL2011-01-28T07:35) was observed on the western limb, observed simultaneously by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Solar TErrestrial RElations Observatory (STEREO). This observation provides the heights of these emissions directly, limited only by the limb references for the two spacecraft, with almost no projection uncertainty. We report the results of this analysis and discuss our findings in terms of present models of particle acceleration and energy transport in the impulsive phase.
A GOES-class X1.4 flare from the NOAA active region 11520 was observed with Hinode/XRT on 12-July, 2012. This region showed sigmoidal structure, which was developed between a group of two major positive-polarity sunspots and an area of small scattered sunspots with negative-polarity. In the course of this flare, a long flare ribbon (typically observed in the light of 304A or H-alpha) was formed on the negative side of the polarity inversion line, while its positive counter part appeared to be concentrated in a brightening around one of the major sunspots. Two notable phenomena were observed associated with this event: (1) Soft X-ray brightening was observed at the foot points of the post flare loops which started to grow ~30 minutes later. (2) Strong and narrow extension of dark lanes (or ejections of low-density plasma) were observed from the both ends of the sigmoid loops at the maximum phase of the flare. As neither of these phenomena are previously reported, proper description and explanation need to be given.
Understanding electron acceleration in solar flares requires X-ray studies with greater sensitivity and dynamic range than are available with current solar hard X-ray observers (i.e. the RHESSI spacecraft). RHESSI employs an indirect Fourier imaging method that is intrinsically limited in dynamic range and therefore can rarely image faint coronal flare sources in the presence of bright footpoints. With greater sensitivity and dynamic range, electron acceleration sites in the corona could be studied in great detail. Both these capabilities can be advanced by the use of direct focusing optics. The recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload demonstrates the feasibility and usefulness of hard X-ray focusing optics for observations of solar hard X-rays. FOXSI features grazing-incidence replicated nickel optics made by the NASA Marshall Space Flight Center and fine-pitch silicon strip detectors developed by the Astro-H team at JAXA/ISAS. FOXSI flew successfully on November 2, 2012, producing images and spectra of a microflare and performing a search for nonthermal emission (4-15 keV) from nanoflares in the quiet Sun. Nanoflares are a candidate for providing the required energy to heat the solar corona to its high temperature of a few million degrees. A future satellite version of FOXSI, featuring similar optics and detectors, could make detailed observations of hard X-rays from flare-accelerated electrons, identifying and characterizing particle acceleration sites and mapping out paths of energetic electrons as they leave these sites and propagate throughout the solar corona.
Where particle acceleration and plasma heating take place in relation to magnetic reconnection is a fundamental question for solar flares. We present here analysis of an M7.7 flare on 2012 July 19 observed by SDO/AIA and RHESSI that sheds new light on this question (Liu, Chen, & Petrosian, 2013, ApJ). Bi-directional outflows in forms of plasmoid ejections and contracting cusp-shaped loops originate between an erupting flux rope (Patsourakos et al. 2013, ApJ) and underlying flare loops at speeds of typically 200-300 km/s up to 1050 km/s. These outflows are associated with spatially separated double coronal X-ray sources with their centroid separation decreasing with energy. The highest temperature is located near the nonthermal X-ray loop-top source, well below the original heights of contracting cusps near the inferred reconnection site. These observations suggest that the primary loci of particle acceleration and plasma heating are in the reconnection outflow regions, rather than the reconnection site itself. We stress that models with this ingredient were proposed long ago (e.g., Forbes & Priest 1983) and backed by recent numerical simulations (e.g., Drake & Swisdak 2012), but solid observational evidence as presented here has been lacking. In addition, there is an initial ascent of the X-ray and EUV loop-top source prior to its recently recognized descent, which we ascribe to the interplay among multiple processes including the upward development of reconnection and the downward contractions of reconnected loops. The impulsive phase onset coincides with the rapid speed increases of the upward plasmoids, the individual loop shrinkages, and the overall loop-top descent, suggestive of an intimate relation of the energy release rate and the reconnection outflow speed.
We investigate the solar cycle variation of the high-degree mode frequencies obtained through the ring-diagram technique. The analysis covers the descending phase of solar cycle 23, the extended minimum period between cycle 23 and 24 and ascending period of cycle 24. Using both GONG and HMI data, we will report on how the frequency shifts measured relative to the spatial and temporal average correlate with activity index.
Helioseismology can be an important tool for understanding the formation of active regions. This poster describes the design of a recently completed study, testing whether pre-appearance signatures of solar magnetic active regions were detectable using various tools of local helioseismology. We provide details of the data selection and preparation of samples, each containing over 100 members, of two populations: regions on the Sun which produced a numbered NOAA active region, and a 'control' sample of areas which did not. The seismology is performed on data from the GONG network; accompanying magnetic data from the Michelson Doppler Imager aboard SoHO are used for co-temporal analysis of the surface magnetic field. Samples are drawn from 2001--2007, and each target is analyzed for 27.7hr prior to an objectively determined time of emergence. We describe known sources of bias and the approaches used to mitigate them. Examining the average ensemble differences between the two populations, we describe significant and surprising differences between our samples in both quantities determined from helioseismology and from surface magnetic fields. This work was supported by NASA contract NNH07CD25C.
We present acoustic travel-time measurements and their center to limb variations. Observational height difference across the solar disk is one of the main sources of spatial variations of travel-time differences used in time-distance helioseismology to measure solar subsurface flows. Using GONG and MDI data we will estimate extra time difference due to this systematic artifact. Preliminary results of center-to-limb corrections on meridional flow measurements will be discussed.
On May 9, 2012, the the straylight level of XRT on Hinode suddenly increased, consistent with the appearance of a pinhole in the entrance filter (possibly a micrometeorite breach). The effect of this event is most noticeable in the optical G band data, which shows an average light excess of ~30%. However, data in several of the X-ray filters is also affected, due to low sensitivity 'tails' of their filter responses into the visible. Observations taken with the G band filter but with the visible light shutter (VLS) closed show a weak, slightly shifted, out-of-focus image, revealing the leaked light. The intensity of the leak depends on telescope pointing, dropping strongly for images taken off-disk. By monitoring light levels in the corners of full-Sun Ti-poly filter images, we determine the approximate time of the event: ~13:30 UT. We use pairs of images taken just-before and after the filter breach to directly measure the leakage in two affected X-ray filters. We then develop a model using a scaled, shifted, and smoothed versions of the VLS closed images to remove the contamination. We estimate the uncertainties involved in our proposed correction procedure. This research was supported under NASA contract NNM07AB07C for Hinode XRT.
Local correlation tracking methods are often used to measure displacement, deformation, strain, and flow in varying fields of study. The Fourier Local Correlation Tracking (FLCT) technique described by Fisher & Welch (2007) is used to track flows on varying scales observed by the Multi-Order Solar EUV Spectrograph (MOSES), a slitless spectrograph imaging in narrowband He II 304 Å. The high spatial resolution (0.6”) and wide field of view (10’ x 20’) combined with the cotemporal spectral data obtained by the MOSES instrument provides a unique range of transition region features and spectral information that may be analyzed using FLCT techniques. Results are reported using FLCT to evaluate small scale flows. The feasibility of extending the technique to include larger scale motions and analysis of spectral information is also considered.
Sunspots usually appear in a group which can be classified by certain morphological criteria. For example, the McIntosh classification has been used to set up solar flare forecast models. It is noted that the McIntosh classification evaluated by a computer and human inspection are often disagreed with each other. Therefore it is necessary to determine the characteristics of sunspot groups calculated by a computer and directly use them for solar flare forecast. We are developing an automatic program to analyze sunspot groups by the hierarchical clustering method using the minimum spanning tree. In this study, a graph consists of sunspots and their geometrical connectivity in the graph theory. The minimum spanning tree is a subset which has the minimum total weight in the graph. The hierarchical clustering method is a suitable technique for the detection of sunspot groups, which is independent of a starting point. We compute not only basic morphological characteristics but also the minimum spanning tree parameters (e.g. depth and degree) centered from the largest spot in a group. We discuss the characteristics of a sunspot group for solar flare forecast.
The Multi-Order Extreme Ultraviolet Spectrograph (MOSES) forms images of the transition region at HE II 30.4 in three spectral orders. Subtle differences between these images encode line profile information. However, differences in instrument point-spread function (PSF) in the three orders lead to non-negligible systematic errors in the retrieval of the line profiles. We describe a technique for equalizing the PSFs, and provide numerical verification of the technique's validity.
Since its first presentation as a demo at the 220th American Astronomical Society Meeting, the Solar Dynamics Observatory (SDO) Content-Based Image-Retrieval (CBIR) system has been open for public usage since December 2012. Incorporating the valuable feedback gathered at the AAS meeting, as well as working closely with solar physicists from Montana State University, this first version of our system provides similar image search capability for the SDO image data repository. In this work we present an overview of the system capabilities, architecture, and future improvements. We also present practical search examples, basic usage instructions, and some of the science data that can be extracted from our system. This work aims to gather more feedback on the system usability and functionality while making the community aware of a promising new tool for exploring SDO data.
The coalignment of solar images from various observational instruments is an important step to study solar features/phenomena when using data acquired from multiple telescopes or wavelengths. We have been working on the calibration of XRT/Hinode to get coalignment parameters, i.e., plate scale, roll angle in the plane of the sky, and pointing information. We are developing a table of the parameters for every single XRT image, so that all users of XRT data can easily utilize our results for their coalignment. There are three different approaches to get the XRT pointing information: (1) applying the cross correlation technique to the X-ray images from XRT and the EUV images from AIA/SDO (2) combination of solar limb fitting and cross correlation between XRT full and partial disk images (3) using outputs from the sun sensor (Ultra Fine Sun Sensor, UFSS) on-board Hinode We discuss the advantages and disadvantages of each method and estimate the errors through the cross comparison. In preparing our table of coalignment coefficients we combine the strengths of each of these methods to provide the most reliable resource. We also discuss the accuracy of the plate scale of X-ray data from XRT, which can be derived from the comparison between XRT and AIA images.
We present an automatic technique to detect and characterize eruptive events (EEs), e.g. prominence eruptions and surges, using SDO/AIA 304 Å images. The technique works as follows. 1) The SDO 304 Å images are polar-transformed for easy handling of the outward motion of EEs and for saving computer resources. 2) The transformed images are divided by a background map, which is determined as the minimum intensity of each pixel during 24 hours. 3) The EEs are defined as a region in the ratio maps with pixels having a ratio >2. Because a stationary prominence has relatively high background, the prominence is detected only when it moves. 4) Pattern recognition is performed to separate different EEs at different locations. 5) In successive images, two EEs with more than 50% of pixels overlapping are considered to be the same EE. 6) If the height of an EE increases monotonically in 5 successive images, we consider it as a reliable eruption. The technique detects 1428 prominence eruptions and 1921 surges from 2010 May to 2012 December. The locations of PEs identified by this technique clearly indicated decayed onset of the maximum phase in the south with respect to the north. This work was supported by NASA Living with a Star TR&T program
The SWAMIS-EF emerging magnetic flux detection algorithm has been operating in the SDO/HMI data pipeline since 2011. We present an update on the status of the module and improvements to the algorithm since it began reporting Emerging Flux HEK events. We will show detailed examples of emerging flux events, and provide some summary information. Finally, we will provide an update on the state of SWAMIS magnetic feature tracking for full-disk, full-resolution HMI line-of-sight magnetograms.
We study the evolution of the observed photospheric magnetic field and the modeled global coronal magnetic field during the past 3 1/2 solar activity cycles observed since the mid-1970s. We use synoptic magnetograms and extrapolated potential-field models based on longitudinal full-disk photospheric magnetograms from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term Investigations of the Sun (SOLIS) vector spectro-magnetograph (VSM), the spectro-magnetograph and the 512-channel magnetograph instruments, and from the U. Stanford's Wilcox Solar Observatory. The associated multipole field components are used to study the dominant length scales and symmetries of the coronal field. Of the axisymmetric multipoles, only the dipole and octupole follow the poles whereas the higher orders follow the activity cycle. All non-axisymmetric multipole strengths are well correlated with the activity cycle. The axial dipole and octupole are the largest contributors to the global field except while the polar fields are reversing. This influence of the polar fields extends to modulating eruption rates. According to the Computer Aided CME Tracking (CACTus), Solar Eruptive Event Detection System (SEEDS), and Nobeyama radioheliograph prominence eruption catalogs, the rate of solar eruptions is found to be systematically higher for active years between 2003-2012 than for those between 1997-2002. This behavior appears to be connected with the weakness of the late-cycle 23 polar fields as suggested by Luhmann. We see evidence that the process of cycle 24 field reversal is well advanced at both poles.
Ideal MHD simulations have shown that the twist of the magnetic flux rope before emergence plays an important role in the coherency of the emerged magnetic structures. Recently, with more realistic simulations with turbulent convection, it is found that magnetic structures can form at the photosphere from emergence of uniform magnetic fields. The discrepancy therefore leads to a controversial question that whether the twist exists before the emergence or is formed afterwards by surface flows. In light of this, we carry out simulations on the emergence of untwisted flux rope from the convection zone into the corona, using more realistic treatment of the thermodynamic processes in the solar interior and the outer atmosphere. In our coupled simulations, we study the interaction between the convective motion and the magnetic fields and also the formation of coronal structures in comparison with observations.
Magnetic reconnection frequently occurs at and around magnetic nulls: locations where the magnetic field strength equals zero. While theoretical models and simulations of magnetic reconnection often assume that the magnetic field null is co-located with a flow stagnation point, the introduction of asymmetry typically leads to flow across the magnetic null. We derive an exact expression for the three dimensional motion of a magnetic null point in a smoothly varying magnetic field. We define xn as the position of a null, U?dxn/dt as the null's velocity, and M as the Jacobian matrix of the magnetic field at the null. By using Faraday's law and evaluating the convective derivative of the magnetic field at xn with velocity U, the velocity of the null is given by U=M-1?×E. This expression is independent of Ohm's law. For resistive magnetohydrodynamics with uniform resistivity ?, this reduces to U=V(xn)-?M-1?2B. This indicates that any difference between the plasma flow velocity at the null and the velocity of the null itself is due to resistive diffusion of the magnetic field. Null points must diffuse in and out of existence. Null-null pairs first appear (or disappear) as a single degenerate null with singular M, and then instantaneously move apart (or together) infinitely fast. However, the motion of separators cannot be described using solely local parameters because the identification of a particular magnetic field line as a separator may change due to non-ideal behavior at another location.
In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field balanced by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset or reconnection that is driven by the buildup of magnetic shear. In MHD simulations, the application of a magnetic field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial and indicates necessity of a true multiscale model for such processes in the Solar environment. The field must be sheared self-consistently/ indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we discuss methods for applying a velocity shear perpendicular to the plane of reconnection for periodic and nonperiodic systems.
We investigate the formation and evolution of coronal flux ropes created by the dynamical emergence of convection zone magnetic fields into various pre-existing coronal magnetic fields. While we found earlier that 2D dynamical flux emergence is not effective at creating coronal flux ropes, either eruptive or stable, we find that in 3D the results are dramatically different. We show that with a dipolar coronal field in a non-reconnecting configuration, a 3D emerging flux rope can form into a stable, prominence-like twisted coronal flux rope. In contrast, we show that when the coronal dipole field is in a reconnecting configuration, a 3D emerging flux rope will reconnect with it to form a breakout quadrupolar field, and then an erupting flux rope. We therefore conclude that, while 2D flux emergence is not an effective mechanism for eruptive flux rope formation, 3D flux emergence is an effective way to simultaneously create a breakout quadrupolar field and to emerge the magnetic shear needed to drive a breakout coronal mass ejection.
We report on observations of spatially-separated, nearly simultaneous small-scale magnetic flux emergence seen in SDO/HMI magnetograms. In the prototypical event, two pairs of bipoles emerge with a nearly identical north-south orientation, the beginning of the separate emergence events begins within 3 hours of each other, and the distance between the pair is approximately twice the bipolar separation distance. This suggests a common subsurface origin. We present a detailed analysis of this emergence event and the associated evolving coronal magnetic field topology, show other examples of similar events, and present a preliminary statistical analysis of the likelihood of such events occurring by chance.
Joy's law governs the tilt of Active Regions (ARs) with respect to their absolute heliographic latitude. Together with Hale's law of hemispheric polarity, it is essential in constraining solar dynamo models. However, previous studies on Joy's law show only a weak positive trend between AR tilt angles and latitudes. In this study, we are focusing on the time dependence of Joy's law, for the cases of emerging ARs of Solar Cycle 24. We selected 40 ARs that emerge on the East hemisphere, effectively maximizing the observing time for each AR. Then, by converting the helioprojective maps into heliographic, we determine the geometrical as well as the magnetic-flux-weighted centroids for each emergence case. That way we are able to track the temporal evolution of their physical properties, including locations, fluxes of positive and negative polarities, as well as the tilt angles of these regions in a continuous manner until emergence stops and the ARs assume their final state.
The Helioseismic and Magnetic Imager (HMI) on SDO has measured magnetic field, velocity, and intensity in the photosphere over the full disk continuously since May 2010 with arc-second resolution. Scalar images are measured every 45 seconds. From these basic observables the pipeline automatically identifies and tracks active regions on the solar disk. The vector magnetic field and a variety of summary quantities are determined every 720s in these tracked Space-weather HMI Active Region Patches (SHARPS). Synoptic and synchronic maps are constructed daily and after each Carrington Rotation Most data products are available with definitive scientific calibration after a few day deal at and in a quick-look near-real-time version a few minutes after the observations are made. Uncertainties are determined for the derived products. All of the magnetic field products along with movies and images suitable for browsing are available at http:://Hmi.stanford.edu/magnetic. Other products, e.g. coronal field over active regions, can be computed on demand.
New magnetic flux even in very small active regions appears as a succession of tiny bipolar magnetic fields that successively and concurrently appear in tight clusters. These smallest observable bipoles were initially called “elementary bipoles” when first seen in videomagnetograms from the Big Bear Solar Observatory (Martin, S. F. 1990, “Elementary Bipoles of Active Regions and Ephemeral Active Regions” Societa Astronomica Italiana, Memorie 61, 293). The magnetic flux of each pole of elementary bipole is approximately the same and measures 1018 Mx or less depending on both the spatial resolution and sensitivity of the magnetograph with which the measurements are made. The two poles initially occur very close together and rapidly move in opposite directions with a typical speed of 3 km/sec. The elementary bipoles within a cluster tend to emerge with similar orientations. The most common orientation of the elementary bipoles at any given time determines the “orientation” of a whole simple bipolar region. In this paper we illustrate and compare 6 clusters of elementary bipoles during the development of a large active region less than 2 days old when observed in H? at the Dutch Open Telescope along with HMI/SDO. Each cluster of elementary bipoles behaves like a single simple bipolar active region. However the clusters are so close together that the magnetic flux of each bipolar cluster merges or cancels with adjacent clusters. The study of elementary bipoles provides a means of simplifying our understanding of the development of complex active regions depending on both the spatial resolution and sensitivity of the magnetograph with which the measurements are made.
The effects of varying the latitudinal extent and peak flow speed of a simulated polar counter-cell are determined by studying the resulting duration and intensity of the activity cycle.
We use daily full-disk vector magnetograms from Vector Spectromagnetograph (VSM) on Solar Optical Long-term Investigations of the Sun (SOLIS) system to synthesize the first Carrington maps of the photospheric vector magnetic field. We describe these maps and make a comparison of observed radial field with the radial field estimate from LOS magnetograms. Further, we employ these maps to study the hemispheric pattern of current helicity density, Hc, during the rising phase of the solar cycle 24. Longitudinal average over the 23 consecutive solar rotations shows a clear signature of the hemispheric helicity rule, i.e. Hc is predominantly negative in the North and positive in South. The hemispheric pattern for individual Carrington rotations is statistically weak, consistent with previous studies of active regions’ helicity. Although our data include the early phase of cycle 24, there appears no evidence for a possible (systematic) reversal of the hemispheric helicity rule at the beginning of cycle as predicted by some dynamo models. Further, we compute the hemispheric pattern in active region latitudes (–30 ? ? ? 30) separately for weak (100< |Br| <500 G)and strong (|Br| >1000 G) radial magnetic fields. We find that while the current helicity of strong fields follows the well-known hemispheric rule (i.e., ?.Hc < 0), Hc of weak fields exhibits an inverse hemispheric behavior (i.e., ?.Hc > 0) albeit with large statistical scatter.
Apparent cyclic and secular changes of sunspot umbral intensities and magnetic field strengths have been reported for many decades. Monthly measurements since 1998 show changes that have been interpreted as a decline in sunspot vigor that, if continued, may lead to very few visible sunspots in forthcoming cycles (Livingston, Penn, and Svalgaard 2012, ApJ 757, L8). This dramatic notion is controversial (e.g. Nagovitsyn, Pevtsov, and Livingston 2012, ApJ 758, L20), and additional observational evidence is needed based on a minimum of interpretational steps and selection effects. The SOLIS vector spectromagnetograph has recorded photospheric spectra around 630.2 nm over the full solar disk daily since late 2003 with spatial and spectral pixel dimensions of about one arc second and 2.3 pm. We fit the unpolarized intensity spectra in sunspots with a simple Zeeman triplet model using a single field strength, which works well if the field strength exceeds about 2 kG and the individual spectra are not affected by strong Doppler and/or Zeeman variations. The derived total field strengths may be compared with independent spectrograph-based measurements from NSO, Hinode/SP, and Mt. Wilson and with filter-based measurements from SoHO/MDI and SDO/HMI. NSO plans to implement this reduction for all suitable archived SOLIS spectra and to continue daily umbral field strength measurements as cycle 24 proceeds. Preliminary comparisons show good agreements in some cases and systematic differences in others.
The prediction and description of the conditions throughout the heliosphere relies today mostly on potential field source surface (PFSS) and magnetohydrodynamics coronal and heliospheric models. Carrington synoptic maps are produced from individual magnetograms and used as the primary drivers for these models. However, the uncertainties on the flux distribution across synoptic maps have never been included in the models. As the measure of uncertainties, we produced synoptic spatial variance (1-sigma standard deviation) maps derived from the distribution of pixel values in the sky magnetograms that contribute the to average flux in each bin of the final Carrington map. Each variance-map is then used to generate a series of Carrington maps where the value of each bin differs, randomly, from the original value by up to 3-sigma. We discuss here how the uncertainty in the Carrington map affects the location of neutral lines and the footpoint locations of the open-field, the model coronal holes, determined from a standard PFSS model. In this preliminary investigation we studied two distinct periods, corresponding to minimum and maximum of solar activity. We show that the variance in the derived synoptic maps does not affect significantly the shape of neutral line or general location of coronal holes. The position of neutral lines and boundaries of coronal holes can be shifted by as much as 5 degrees in some locations.
This study seeks to further quantify the relationship between the boundaries of coronal holes and open magnetic field regions. Utilizing the combined observations of the SDO:AIA and STEREO:EUVI A/B instruments, nearly full coverage of the solar surface in several EUV filters is available. Using this data we have devised a routine to define global observations of coronal hole boundaries at high cadence. For comparison, several methods of global coronal magnetic field extrapolation were considered, both potential and non-potential. We considered both a direct spatio-temporal comparison of boundaries as well as associated magnetic flux quantities.
Cycle 24 marks a change in solar activity in recent times. It was preceded by the longest and deepest minimum in 100 years and is the weakest cycle we have observed with modern instrumentation. It also shows a remarkable asymmetry between the two hemispheres. Already in cycle 23 a lack of very large spots was noticeable, but in cycle 24 spots of all sizes have been scarce, especially in the southern hemispheres. We compare magnetic activity during the last three solar cycles by examining various activity indicators from the solar photosphere to the corona. We find that the decrease in the number of large spots in cycle 23 was mainly responsible for the decrease in total sunspot area during cycle 23 but we do not find evidence for a systematic decrease in sunspot contrast with time. Not only large spots were less frequent in cycle 23 but they appeared late in the cycle and at generally lower latitudes. This resulted in weak polar fields at the end of cycle 23. At present, the southern pole is reversing while the northern one has already reversed sign. The time delay in the two polar reversals has been very noticeable in coronal activity in the number of polar crown eruptions in the two hemispheres.
Properties of solar vector magnetic fields can be determined by the inversion of polarization spectra. It is therefore important to have accurate inversion methods. Milne-Eddington inversions, used almost exclusively in the photosphere, assume a thin, flat atmosphere and are one of the most widely used inversion techniques. To investigate the potential weaknesses of parameterizing a stratified atmosphere using a single set of properties, we examine the consequences of using a Milne-Eddington inversion to invert spectra of complex atmospheres. Han Uitenbroek's Rybicki-Hummer radiative transfer and chemical equilibrium code was used to generate a series of one-dimensional model atmospheres with predetermined magnetic field configurations. Atmospheres at the quiet Sun temperature contained magnetic fields with strengths up 3000 G and inclination and azimuthal angles from 0 to 180 degrees. We examined the Stokes profiles of the Fe 15648.5 line, which with a Landé g-factor of 3.0 is very sensitive to the magnetic field. Using a simple Milne-Eddington inversion code, we examined the ranges in which the code accurately parameterized the magnetic field. To investigate the confidence intervals associated with the inverted parameters, we used the BayesME code developed by Andres Asensio Ramos. We discuss the key assumptions and limitations of a Milne-Eddington inversion.
The X-ray image of a partially-occulted solar flare, one occurring just behind the limb of the Sun, can have a sharply defined X-ray edge resulting from attenuation in the atmosphere of the quiet Sun in the foreground. Our analysis makes use of RHESSI's direct measurement of image Fourier visibilities, and we estimate that the ultimate precision of the limb height will on the order of the photospheric scale height in the region of dominant absorption. This occurs at an altitude depending on the X-ray photon energies used for the measurement, but generally in the upper photosphere and chromosphere. We give a preliminary report on analysis of one suitable event, the flare SOL2002-04-04T15:32 (M6.1), where we find a clean signature of this attenuation up to the RHESSI hard X-ray range 12-25 keV. At this energy Compton scattering begins to dominate the attenuation, greatly reducing the model-dependence of the result; at lower photon energies photoelectric absorption becomes more important. These data determine the physical altitude of the mean atmospheric density, with minimal model dependence, and therefore provide an independent calibration of the atmospheric height scale.
The minimum-energy state for the magnetic field above the solar photosphere is current-free. Since current is proportional to the magnetic field's curl, the minimum energy state is curl-free, and can therefore be represented as the gradient of a scalar potential. In addition, the divergence-free condition on the magnetic field implies this scalar potential obeys Laplace's equation. An appropriate boundary condition must be specified to determine the potential. Given a measurement of the full magnetic vector at the photosphere, it is possible to employ either Neumann or Dirichlet boundary conditions there. Typically, the Neumann condition is applied. Since either condition fully specifies the three-dimensional vector magnetic field, either choice will, in general, be inconsistent with some aspect of the observed field on the boundary, due to the presence of both currents and noise in the observed field. We present a method to derive a potential field that minimizes the integrated square of the residual between both boundary conditions, and show one way to incorporate weighting by spatially uniform measurement uncertainties in the minimization. We demonstrate each approach using HMI vector magnetic field observations of AR 11158. Residual discrepancies between the observed and potential fields are significantly larger than empirically determined noise levels, and can be interpreted as evidence of horizontal photospheric currents. The data provide clues about properties of these currents, but determining their spatial distribution will require additional constraints, e.g., simplifying assumptions about photospheric magnetic structure or observational input beyond single-height magnetograms. We also find that the energies of potential fields computed in different ways are significantly different --- by nearly 1e+33 ergs in some cases. This has substantial implications for estimates of the free magnetic energy in coronal field models that is available to power flares and coronal mass ejections (CMEs.)
Photospheric magnetic surface diffusion is an important constraint for the solar dynamo. The HMI Active Region Patches (HARPs) program automatically identify all magnetic regions above a certain flux. In our study we measure the moments of ARs that are no longer actively emerging and can thereby give us good statistical constraints on photospheric diffusion. We also present the diffusion properties as a function of latitude, flux density, and single polarity (leading or following) within each HARP.
We study the role of rotating sunspots in relation to the evolution of various physical parameters characterizing the non-potentiality of the active region (AR) NOAA 11158 and its eruptive events using the magnetic field data from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. From the evolutionary study of HMI intensity and AIA channels, it is observed that the AR consists of two major rotating sunspots, one connected to a flare-prone region and another with coronal mass ejection (CME). The constructed space–time intensity maps reveal that the sunspots exhibited peak rotation rates coinciding with the occurrence of major eruptive events. Further, temporal profiles of twist parameters, namely, average shear angle, ?av, ?best, derived from HMI vector magnetograms, and the rate of helicity injection, obtained from the horizontal flux motions of HMI line-of-sight magnetograms, correspond well with the rotational profile of the sunspot in the CME-prone region, giving predominant evidence of rotational motion causig magnetic non-potentiality. Moreover, the mean value of free energy from the virial theorem calculated at the photospheric level shows a clear step-down decrease at the onset time of the flares revealing unambiguous evidence of energy release intermittently that is stored by flux emergence and/or motions in pre-flare phases. Additionally, distribution of helicity injection is homogeneous in the CME-prone region while in the flare-prone region it is not and often changes sign. This study provides a clear picture that both proper and rotational motions of the observed fluxes played significant roles in enhancing the magnetic non-potentiality of the AR by injecting helicity, twisting the magnetic fields and thereby increasing the free energy, leading to favorable conditions for the observed transient activity.
We use observations from the Solar Aspect Sensor (SAS) of RHESSI to characterize the latitude dependence of the temperature of the photosphere near the solar limb. Previous observations had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as a function of position angle. We further mask these images against EUV images (SOHO/EIT 284A for older data and SDO/AIA for more recent data) in order to eliminate magnetic regions. We present results from our new analysis which shows significantly larger signals of latitude-dependent temperature variations than what has been presented earlier in our preliminary analysis and interpretation.
We have measured the meridional motions of magnetic elements observed in the photosphere over sunspot cycles 23 and 24 using magnetograms from SOHO/MDI and SDO/HMI. Our measurements confirm the finding of Komm, Howard, and Harvey (1993) that the poleward meridional flow weakens at cycle maxima. Our high spatial and temporal resolution analyses show that this variation is in the form of a superimposed inflow toward the active latitudes. This inflow is weaker in cycle 24 when compared to the inflow in 23, the stronger cycle. This systematic modulation of the meridional flow should also modulate the amplitude of the following sunspot cycle through its influence on the Sun’s polar fields. The observational evidence and the theoretical consequences (similar to those of Cameron and Schussler (2012)) will be described. Komm, Howard, and Harvey (1993) Solar Phys. 147, 207. Cameron and Schussler (2012) Astron. Astrophys. 548, A57.
We have made a comparison of the CME-associated shock propagation based on the WSA-ENLIL model with three cone types using 29 halo CMEs from 2001 to 2002. These halo CMEs have cone model parameters as well as their associated interplanetary (IP) shocks. For this study we consider three different cone types (an asymmetric cone model, an ice-cream cone model and an elliptical cone model) to determine 3-D CME parameters (radial velocity, angular width and source location), which are the input values of the WSA-ENLIL model. The mean absolute error (MAE) of the arrival times for the asymmetric cone model is 10.6 hours, which is about 1 hour smaller than those of the other models. Their ensemble average of MAE is 9.5 hours. However, this value is still larger than that (8.7 hours) of the empirical model of Kim et al. (2007). We will compare their IP shock velocities and densities with those from ACE in-situ measurements and discuss them in terms of the prediction of geomagnetic storms.
The theory of Thomson scattering, while well understood when applied to features observed in the corona, has been misunderstood by many when applied to features in the heliosphere. The confusion arises when assumptions applied to analyzing coronagraph data are extended to heliospheric imagers, where the consequences of the theory are somewhat different. Two crucial problems are the assumption of close proximity to the Thomson surface and detectability being governed by the background F and K corona. We demonstrate that these assumptions do not apply to heliospheric imagers. Further, we explore polarized heliospheric imaging in the context of transient feature detection. We show that not only are features detectable above the background, but also that additional information on transients such as coronal mass ejections can be extracted from a combination of polarized and unpolarized images than from unpolarized imaging alone.
We performed a case study on the effect of CME interactions on SEP intensity for the 2012-March-07 SEP event. The 2012 March 07 SEP event had the second largest intensity during solar cycle 24. The SEP/ESP peak intensities peaked at 1500/6000 pfu. The event was associated with a double X-ray flare and two CMEs in quick succession. In soft X-rays, the flares reached peak flux levels of X5.4 (00:02 UT) and X1.1, respectively, from AR1429 (N18E31). The flare peaks were at ~00:24 and ~01:14 UT, while the onset times were 00:02 UT and 01:05 UT. The associated CMEs were very fast: 2376 km/s (CME1) and 2203 km/s (CME2) and appeared < 1 hour apart. Two distinct type II radio bursts were detected in the decameter-hectometric (DH) spectrum observed by the Wind/WAVES experiment. The interaction of two CMEs was clearly seen from STEREO COR2 B movie from West limb around 01:54 UT, with enhanced signature in DH type II spectrum around the same time. The two CMEs arrived at 21.5 Rs (inner boundary of ENLIL) at 01:55 UT and 02:40 UT, respectively, with flux-rope fitted propagation directions of N17E27 and N00E17. Two ENLIL runs were performed: 1) only CME1 was inserted at 21.5Rs and 2) both CME1 and CME2 were inserted in the simulation to study the effect of CME interactions. Comparing the results of the two runs, we found that both the shock intensity and shock speed of Run2 were higher than Run1, suggesting that the CME interaction have not only enhanced shock intensity but also caused higher speed, therefore resulting in larger SEP intensity. This work was supported by NASA Living with a Star TR&T program
We studied the source regions of 12 solar energetic particle (SEP) events seen between 2010 August and 2012 January at STEREO-A, B and/or Earth (ACE/SOHO/GOES), when the two STEREO spacecraft were separated by about 180 degrees. All events were associated with flares (C1 to X6) and fast coronal mass ejections (CMEs) and, except for one, accompanied by type II radio bursts. We have determined the arrival times of the SEPs at the three positions. Extreme ultraviolet (EUV) waves, observed in the 195 and 193 angstrom channels of STEREO and SDO, are tracked across the Sun to determine their arrival time at the photospheric source of open field lines connecting to the spacecraft. There is a good correlation between the EUV wave arrival times at the connecting footpoints and the SEP onset times. The delay time between electron onset and the EUV wave reaching the connecting footpoint is independent of distance from the flare site. The proton delay time increases with distance from the flare site. In three of the events secondary flare sites may have also contributed to the wide longitudinal spread of SEPs.
Coronal Mass Ejection arrival prediction at the Earth is one of the most important topics in space weather research. To quickly and accurately predict the arrival of CME ejecta and also their associated shock waves, different data sets must be used. Using measurements of the CME ejecta from STEREO and SOHO and comparing them to the aerodynamic drag model, which assumes that the major fact in CME kinematics is given by the CME initial velocity and how it is effected by the ambient solar wind speed, we provide understanding into how the CME ejecta evolves as the CME propagates into the heliosphere. Using measurements of the shock wave we can also learn more about how the mechanisms by which the shock front is driven by the CME and see how the standoff distance between the shock and CME ejecta evolves as the CME moves further into the interplanetary space. As CME driven shocks are just as capable as CMEs of producing geomagnetic storms, this is an important factor for space weather prediction. By comparing observations remote-sensing observations with in-situ signatures at ACE over a significant sample of events, our study can possibly lead to a simple model that both captures the physics of the CME-shock interaction and also allows for quick predictions with the maximum possible advance time.
Flaring active regions, as observed by Hinode's XRT and SOHO's MDI are associated with the power of the solar wind as observed with the ACE spacecraft at 1AU. We recover an empirical relationship between solar wind power and magnetic flux at the Sun's surface that is almost identical to a well-known scaling between magnetic flux and the power required for coronal heating. In addition to the solar wind flux, energetic particles are also correlated as seen by the ACE spacecraft.
The solar cycle and its associated magnetic activity are the main drivers behind changes in the interplanetary environment and the Earth's upper atmosphere. These changes have a direct impact on the lifetime of space-based assets and can create hazards to astronauts in space. In recent years there has been an effort to develop accurate solar cycle predictions (with aims at predicting the long-term evolution of space weather), leading to nearly a hundred widely spread predictions for the amplitude of solar cycle 24. In this presentation we show how cycle predictions can be made more accurate if performed separately for each hemisphere, taking advantage of information about both the dipolar and quadrupolar moments of the solar magnetic field. Additionally, by extending the relationship between polar flux at solar minimum and the amplitude of the next cycle to encompass a full century, we demonstrate the power of predictions based on the solar polar field -- paving the way for a new generation of better and more accurate solar cycle predictions.
It is well known that fast and wide CMEs are generally involved in large solar energetic particle (SEP) events and are generally important in affecting the heliosphere. We examined the solar sources of all wide CMEs (width ? 60°) observed by SOHO/LASCO and STEREO/SECCHI in 2011 (ascending phase of solar cycle 24) by confirming the back-sided events using STEREO data. We identified 600 wide CMEs, of which 323 (54%) and 163 (27%) were associated with active region flares and eruptive quiescent filaments, respectively. In 88 cases (15%) only EUV dimmings were observed. The remaining 26 (4%) CMEs were of unknown origin, including 7 stealth CMEs. It is significant that about 1% of all wide CMEs do not have identifiable near-surface signatures. Considering only the front side CMEs, the median X-ray intensity associated with ARs and filament regions were C3.7 and C1.6, respectively. The average speeds of AR and quiescent-filament CMEs were, 537 and 373 km/s. We conclude that some the results of the previous solar cycles can be clarified because of the availability of STEREO data.
We have begun analyzing photometric background-subtracted images from STEREO/SECCHI's HI-1 instrument. The images show a clear 'flocculated' pattern to the solar wind that is reminiscent of more familiar turbulent flows and more uniformly structured than the top of the corona seen with LASCO C-3 or STEREO/SECCHI COR2. We will present initial quantitative results of this analysis, indicating whether the flocculation pattern is produced locally or advected intact from the corona. This bears heavily on the question of the origin of the variable slow solar wind.
We are currently processing the STEREO/SECCHI heliospheric imaging data to separate the starfield, and depositing them into the SDAC and STEREO data repository. We present an overview of the data, the quality, how to use (and how not to overinterpret) them scientifically, and status of the processing project.
The Helioviewer Project enables visual exploration of the Sun and the inner heliosphere for everyone, everywhere via intuitive interfaces and novel technology. Images from the SDO, STEREO, SOHO, PROBA2 and Yohkoh missions are currently available. Users of the Helioviewer Project have made over one million movies and over two million screenshots since detailed (and anonymous) logging of Helioviewer Project usage was implemented in February 2011. These usage logs are analyzed to give a detailed breakdown on user interaction with solar and heliospheric data via Helioviewer Project clients and services. We present summary statistics on how our users are using our clients and services, which data they are interested in, and how they choose to interact with different data sources.
The vector magnetic field pipeline of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectromagnetograph (VSM) is currently undergoing several key upgrades. Improved algorithms for fringe removal, quicklook vector magnetic field estimation, Milne-Eddington inversion, and geometrical image correction have been implemented, resulting in drastically improved data products which the National Solar Observatory (NSO) routinely provides to the solar community. This poster highlights the recent improvements we have made and points the users of our spectropolarimetric and magnetic field data to our new and improved data products.
Over the past two years the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Team made significant improvements to the data products provided to the solar and heliospheric community. Longitudinal photospheric magnetograms were consistently calibrated to form a uniform magnetic flux series from 2003-present. Improvements in image geometric corrections enabled providing the high spectral resolution spectra for each pixel in full disk images taken in Fe 630.2 nm and Ca II 854.2 nm. New treatment of high resolution spectra from the Integrated Sunlight Spectrometer (ISS), led to significant reduction in daily variations of parameters derived from these spectra. Several new data products were developed, including simultaneous photospheric and chromospheric longitudinal magnetograms derived from the core and wings of Ca II 854.2 nm line, and derivation of field strength via fitting non-polarized I-profiles of Fe I 630.2 nm. We present here a summary of recent changes to this database, with particular emphasis on chromospheric longitudinal magnetic field and intensity measurements, photospheric vector magnetic field products, and high-spectral resolution integrated sunlight data. Some significant results derived from the analysis of these data sets will also be presented, while more detailed descriptions about specific topics will be referred to other SOLIS-related studies presented at this meeting.
Python, a free, cross platform, general purpose, high-level programming language, has seen widespread adoption among the scientific community resulting in the availability of a large range of software, from numerical computation (NumPy, SciPy) and machine learning to spectral analysis and visualization (Matplotlib). SunPy is a data analysis toolkit specializing in providing the software necessary to analyze solar and heliospheric datasets in Python. It aims to provide a free and open-source alternative to the IDL-based SolarSoft (SSW) solar data analysis environment. We present the latest release of SunPy (0.3). This release includes a major refactor of the main SunPy code to improve ease of use for the user as well as a more consistent interface. SunPy provides downloading capability through integration with the Virtual Solar Observatory (VSO) and the the Heliophysics Event Knowledgebase (HEK). It can open image fits files from major solar missions (SDO/AIA, SOHO/EIT, SOHO/LASCO, STEREO) into WCS-aware maps. SunPy provides advanced time-series tools for data from mission such as GOES, SDO/EVE, and Proba2/LYRA as well as support for radio spectra (e.g. e-Callisto). We present examples of solar data analysis in SunPy, and show how Python-based solar data-analysis can leverage the many existing data analysis tools already available in Python. We discuss the future goals of the project and encourage interested users to become involved in the planning and development of SunPy.
I report on the current status of the Virtual Solar Observatory (VSO), and specifically on its shortcomings, backlogs, and plans for resolving them, including, but not limited to additional data providers, the aging web GUI, and performance. I solicit suggestions from the community on other improvements, and prioritization of our efforts.
The Virtual Solar Observatory allows searching across many collections of solar physics data, but does not yet allow a researcher to search based on the location and extent of the observation, other than by selecting general categories such as full disk or off limb. High resolution instruments that observe only a portion of the the solar disk require greater specificity than is currently available. We believe that finer-grained spatial searching will allow for improved access to data from existing instruments such as TRACE, XRT and SOT, and well as from upcoming missions such as ATST and IRIS. Our proposed solution should also help scientists to search on the field of view of full-disk images that are out of the Sun-Earth line, such as STEREO/EUVI and obserations from the upcoming Solar Orbiter and Solar Probe Plus missions. We present our current work on cataloging sub field images for spatial searching so that researchers can more easily search for observations of a given feature of interest, with the intent of soliciting information about researcher's requirements and recommendations for further improvements.
The Prominence Magnetometer (ProMag) is a dual-channel, dual-beam, slit-scanning, full Stokes spectro-polarimeter designed by the High Altitude Observatory at the National Center for Atmospheric Research (HAO/NCAR) for the study of the magnetism of solar prominences and filaments. It was deployed in August 2009 at the 40 cm coronagraph of the Evans Solar Facility (ESF) of the National Solar Observatory on Sacramento Peak (NSO/SP). In its standard mode of operation it acquires spectro-polarimetric maps of solar targets simultaneously in the two chromospheric lines of He I at 587.6 nm and 1083.0 nm. Since August 2011 ProMag has operated in “patrol mode” with a dedicated observer. We aim to routinely measure the vector magnetic field in prominences. The electro-optic modulator and polarization analyzer are integrated into a single mechanical unit located at the coude feed of the telescope. This location was necessary for proper co-alignment of the dual beams, but complicates the precise polarimeter calibration necessary to achieve the sensitivity required for prominence measurements (< 10^-3). At this sensitivity, small variations in optical alignment can become significant. We present a calibration method for ProMag, using a polarizer and retarder at coronagraph prime focus. Calibrations are recorded before and after observations. We discuss the success of this method and its limitations.
The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, a confirmed M-class mission of the European Space Agency (ESA) within the Cosmic Vision program scheduled to be launched in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The status of the instrument that will be presented at the Critical Design Review (CDR) later this year will be discussed in this poster.
For over four decades, X-ray, EUV, and UV spectral observations have been used to measure physical properties of the solar atmosphere. During this time, there has been substantial improvement in the spectral, spatial, and temporal resolution of the observations for the EUV and UV wavelength ranges. At wavelengths below 100 Angstroms, however, observations of the solar corona with simultaneous spatial and spectral resolution are limited, and not since the late 1970’s have spatially resolved solar X-ray spectra been measured. The soft-X-ray wavelength range is dominated by emission lines formed at high temperatures and provides diagnostics unavailable in any other wavelength range. Using a novel implementation of corrective optics, the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) will measure, for the first time, the solar spectrum from 6-24 Angstroms with a ~2' pixel size. The culmination of technological advances in grating lithography, mirror fabrication techniques, and camera efficiencies can now be leveraged to build imaging spectrographs similar to Chandra but at a far more reasonable cost. We will demonstrate the singular scientific capabilities of MaGIXS and present the progression of its technological development.
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a VUV spectropolarimeter optimized for measuring the linear polarization of the Lyman-alpha line (121.6 nm). The Lyman-alpha line is predicted to show linear polarization caused by atomic scattering in the chromosphere and modified by the magnetic field through the Hanle effect. The Hanle effect is sensitive to weaker magnetic fields than Zeeman effect, and is not canceled by opposing fields, making it sensitive to tangled or unresolved magnetic field structures. These factors make the Hanle effect a valuable tool for probing the magnetic field in the chromosphere above the quiet sun. To meet this goal, CLASP is designed to measure linear polarization with 0.1% polarization sensitivity at 0.01 nm spectral resolution and 10' spatial resolution. CLASP is scheduled to be launched in 2015.
HIgh resolution EUV spectroscopy is a critical instrumental technique to understand fundamental physical processes in the high temperature solar atmosphere. Spectroscopic observations are used to measure differential emission measure, line of sight and turbulent flows, plasma densities and emission measures. Spatially resolved, spectra of these emission lines with adequate cadence will provide the necessary clues linking small scale structures with large scale, energetic solar phenomena. The necessary observations to determine underlying physical processes and to provide comprehensive temperature coverage of the solar atmosphere above the chromosphere will be obtained by the proposed EUVST instrument for Solar C. This instrument and its design will be discussed in this paper. Progress on the VEry high Resolution Imaging Spectrograph (VERIS) sounding rocket instrument presently under development at the Naval Research Laboratory will also be discussed.
The Miniature X-ray Imager (MiXI) is an ambitious, innovative, small, and fully functional solar X-ray observatory concept designed to fit within a 6U CubeSat platform. MiXI will provide the community with X-ray imaging and spectroscopy of solar flares, but at a small fraction of the cost of a conventional mission. MiXI will observe from 3 to 50 keV. It includes rotation modulation collimators and layered Si/CdTe detectors, providing routine observations of both soft and hard X-ray emission with low background. If selected for funding, MiXI could launch in 2017 to coincide with the launch of Solar Orbiter. In the next solar cycle, coordinated observations between the STIX instrument onboard Solar Orbiter and a future version of MiXI will enable solar flare observation from two vantage points. This will provide new insight into the directivity of flare HXR emission and will allow detailed study of both coronal and footpoint sources in the same flare. These results may have profound implications for theories of flare acceleration processes. We describe here the MiXI concept and its usefulness to the solar and heliophysics communities.
The Rapid Acquisition Imaging Spectrograph Experiment (RAISE) sounding rocket payload is a high speed scanning-slit imaging spectrograph designed to observe the dynamics and heating of the solar chromosphere and corona on time scales as short as 100 ms, with 1 arcsec spatial resolution and a velocity sensitivity of 1-2 km/s. The instrument is based on a new class of UV/EUV imaging spectrometers that use only two reflections to provide quasi-stigmatic performance simultaneously over multiple wavelengths and spatial fields. The design uses an off-axis parabolic telescope mirror to form a real image of the sun on the spectrometer entrance aperture. A slit then selects a portion of the solar image, passing its light onto a near-normal incidence toroidal grating, which re-images the spectrally dispersed radiation onto two array detectors. Two full spectral passbands over the same one-dimensional spatial field are recorded simultaneously with no scanning of the detectors or grating. The two different spectral bands (1st-order 1205-1243Å and 1526-1564Å) are imaged onto two intensified Active Pixel Sensor (APS) detectors whose focal planes are individually adjusted for optimized performance. The telescope and grating are coated with B4C to enhance short wavelength (2nd order) reflectance, enabling the instrument to record the brightest lines between 602-622Å and 761-780Å at the same time. RAISE reads out the full field of both detectors at 5-10 Hz, allowing us to record over 1,500 complete spectral observations in a single 5-minute rocket flight, opening up a new domain of high time resolution spectral imaging and spectroscopy. We present an overview of the project, a summary of the maiden flight results, and an update on instrument status.
The interpretation of astrophysical spectra requires knowledge of the charge state distribution (CSD) of the plasma. The CSD is determined by the rates of ionization and recombination. Thus, accurate electron impact ionization (EII) data are needed to calculate the CSD of the solar atmosphere as well as for other electron-ionized astrophysical objects, such as stars, supernovae, galaxies, and clusters of galaxies. We are studying EII for astrophysically important ions using the TSR storage ring located at the Max Plank Institute for Nuclear Physics in Heidelberg, Germany. Storage ring measurements are largely free of the metastable contamination found in other experimental geometries, resulting in unambiguous EII data. We have found discrepancies of about 10% - 30% between our measured cross sections and those commonly used in CSD models. Because it is impractical to perform experimental measurements for every astrophysically relevant ion, theory must provide the bulk of the necessary EII data. These experimental results provide an essential benchmark for such EII calculations.
Magnetic flux ropes are self-organized, magnetized plasma structures embedded in an ambient medium. Their structure consists of helical field lines which vary in pitch due to the electric current flowing along a background magnetic field.1 Multiple braided flux ropes have been observed in the solar corona, and their unraveling is theorized to be the signature of magnetic reconnection.2 Two flux ropes (L=10 m, A=7 cm2, J=10 amp/cm2) were created in the Large Plasma Device (LAPD) at UCLA (Bo=330 G, no = 1012 cm-3, Te=4eV, Ar). The flux ropes are highly kink unstable, which cause the ropes to twist and oscillate at frequencies associated with shear Alfven waves. Through the use of a six-faced Mach probe, volumetric data was taken to determine the three-dimensional plasma flow. Volumetric b-field information was also obtained through use of a three-axis magnetic probe. The data collected from these probes is laden with Lorentzian pulses, a characteristic of deterministic chaos.3 The flux ropes are shown to twist, interact, then merge; while the plasma flows are shown to spiral around the two flux ropes in a singular O-point. A quasi-separatrix layer (QSL) forms as the flux ropes collide and the magnetic field lines reconnect. The relationship between flow and reconnection sites is explored. 1Gekelman, W. et al. ApJ 753, 131 2Cirtain, J.W. et al. Nature 493, 501-503 (2013) 3Maggs, J.E. et al. Phys. Rev. Lett. 107, 185003 (2011)
Eruption of arched magnetoplasma structures is a fundamental process that drives solar energetic events on wide spatiotemporal scales in the solar atmosphere. The term arched magnetic flux rope (AMFR) is associated with such structures since they carry electrical current which generates a twisted magnetic structure. In the limit of a low electrical-current, the magnetic-field-line-twist becomes small and a magnetic flux rope resembles the structure of a magnetic flux tube. A laboratory plasma experiment has been constructed at UCLA which is capable of generating reproducible AMFR eruptions with a 0.5 Hz repetition rate and recording their spatiotemporal evolution using computer-controlled movable probes (n ~ 1013 cm-3, Te ~ 10 eV, L = 0.5 m, I = 100 A, B ~ 1 kG at footpoints). The experiment has been designed by careful scaling of the relevant solar plasma parameters and the boundary conditions can be controlled to simulate a variety of drive mechanisms that may exist on the Sun (e.g., mass flow vs current flow from the AMFR footpoints, slow vs fast buildup of the magnetic energy in the arch). The AMFR evolves in a large background magnetoplasma (n ~ 1012 cm-3, Te ~ 4 eV, B = 20–100 G). The relative magnitudes of the plasma parameters of the AMFR and the ambient magnetoplasma can be varied. Stereo images of the AMFR evolution are recorded by a fast CCD camera using a variety of pass-band filters. In this presentation, recent experimental results comparing a fast eruption (time scale t < 3. Alfven transit time in the arch) with a slow eruption (time scale t > 100. Alfven transit time in the arch) of the AMFR will be discussed. The highlights of the post-eruption AMFR are low frequency global kink mode oscillations (f ~ 200 kHz) that appear concurrently with high-frequency fast waves (f ~ 5 MHz) in the AMFR. References: (1) S. K. P. Tripathi and W. Gekelman, Phys. Rev. Lett. 105, 075005 (2010) (2) S. K. P. Tripathi and W. Gekelman, Solar Physics, Published online 8 March, 2013 *Work performed at Basic Plasma Science Facility, UCLA and supported by US DOE and NSF
Helioviewer.org enables the simultaneous exploration of multiple heterogeneous solar data sets. In the latest iteration of this open-source web application, Yohkoh SXT joins SDO, SOHO, STEREO, and PROBA2 as a supported data set. A newly enhanced user-interface expands the utility of Helioviewer.org by adding annotations to the imagery. Backed by data from the Heliospheric Events Knowledgebase (HEK), Helioviewer.org can now overlay solar feature and event data (selectively by type and detection method) through the display of interactive marker pins, region outlines, data labels, and information panels. The addition of a size-of-the-Earth indicator provides a sense of the scale to solar and heliospheric features for education and public outreach purposes.
The transit of Venus in June 2012 provided a unique chance to view its atmosphere as we might see that of a transiting Cytherean exoplanet, through scattered and refracted illumination from its parent star. We performed spectroscopy and polarimetry during the transit of Venus focusing on extracting signatures of CO2 absorption of Venus from the solar spectrum. Although the predicted CO2 transmission spectrum of Venus was not particularly strong at 1565 nm, this region of the H-band often used in magnetic-field studies of the Sun's photosphere provides a particularly flat solar continuum with few atmospheric and molecular lines. Observations of Venus were taken throughout first contact and on the solar disk using the Facility InfraRed Spectropolarimeter on the Dunn Solar Telescope at the National Solar Observatory. The transit also provided a unique opportunity to investigate instrumental effects. In this poster we discuss initial results from the transit, including estimates for an exoplanet detection of this kind, preliminary comparison with atmospheric models, and the stray light properties of the instrument. This work was performed in collaboration with the Williams College Venus transit expedition, which was sponsored by Natl Geog/Comm for Research and Exploration.
The Sun-Earth L5 Lagrange point is an ideal location for an operational space weather mission to provide early warning of Earth-directed solar storms (CMEs, shocks and associated solar energetic particles) so the effects on power grids, spacecraft and communications systems can be mitigated. Such missions have been proposed using conventional spacecraft and chemical propulsion at costs of hundreds of millions of dollars. Here we describe a mission that can accomplish the goals at a much lower cost by dividing the payload among a cluster of interplanetary CubeSats that reach orbits around L5 using solar sails. The ascendancy of CubeSats has brought renewed interest in solar sail propulsion because sail area scales directly with spacecraft mass. The concept presented here draws heavily on a NIAC study (Staehle et al., AIAA, 2012) that developed a 6U CubeSat architecture for interplanetary missions. This study allocated 2U for a solar sail; the sail system was based on the Planetary Society’s LightSail-1TM architecture. At a recent workshop on small satellites, hosted by the Keck Institute for Space Studies, a concept was developed for a fractionated Space Weather Base (SWB) at L5. In this concept, a loose formation of CubeSats, each ~6U in size and each carrying a portion of the science payload, can accomplish, at a much reduced cost, many of the goals of a conventional single-spacecraft L5 mission, as described in the 2013 NRC Solar and Space Physics Decadal report. Each of the small ~6U interplanetary CubeSats reaches an orbit around L5 using its own solar sail of approximately 64 m2 which fits in ~2U. Key to the mission is that only one of the CubeSats carries a high-gain antenna and other hardware necessary for sending high-rate science data to Earth. The other CubeSats, in addition to carrying one or two science instruments, carry a much smaller communication system to send the science data to the communication hub and low-rate engineering data to Earth. The SWB-L5 mission can later be expanded incrementally to add new instruments and new objectives by sending additional small spacecraft to the L5 base. The mission described below represents a potential beginning for a permanent space warning system at L5.
The nearby Alpha Centauri triple system has two solar-type stars in a relatively close orbit (20 au separation), and a dim red dwarf companion -- Proxima -- about 10,000 au away, on the Sun-ward side of the group. The heaviest star -- Alpha Cen A -- is a close twin of the Sun. Its slightly less massive companion -- Alpha Cen B -- is a K-type dwarf, and is the closest star thought to host an exoplanet (Earth-sized, but in a much tighter orbit). The close pair has been scrutinized for more than a decade in X-rays by XMM and Chandra, on a semiannual basis since 2003 and 2005, respectively. However, in recent years only Chandra has been able to cleanly separate the pair, which are approaching closest separation on the sky (only a few arcseconds) in their 80-year orbit. For the past 3 years, the HST STIS spectrograph has joined the crowd, also capturing FUV snapshots of the pair every six months. The Alpha Cen stars provide an important complement to long-term studies of the Sun at high energies. The K-star has displayed a clear 8-year cycle in recent years, while the G-star remains mired in a Maunder-like minimum.
Calibrated HMI basic observable quantities have been available for almost 3 years. Higher level standard HMI data products for magnetic fields and helioseismic derived parameters have been available for more than a year. This talk will give a brief status update on the standard products and describe a few groundbreaking findings based on these products.
Several studies have shown that the sunspot areas recorded by the Royal Greenwich Observatory (RGO) between 1874 –1976 are about 40 - 50% larger than those measured by the NOAA/USAF Solar Observing Optical Network (SOON) since 1966. Many possible contributions to this surprisingly large difference have been suggested, but no satisfying explanation has emerged. We show here that, while the two measurement sets provide consistent areas for large spots, the low resolution of the SOON drawings leads to an underestimate of small- spot areas. These are more accurately recorded by the RGO and other programs which use photographic or ccd images. The large number of such small spots is often overlooked; it appears sufficient to explain the reported scale difference. Our explanation suggests that the sunspot blocking of solar irradiance calculated from the RGO areas is over- estimated by approximately 20% because the small spots have low photometric contrast. The smaller SOON areas seem to under-estimate blocking by less than 10 %. Higher accuracy of the blocking time series will require better accounting of the spot area distribution, and a better measurement of the size dependence of sunspot bolometric contrast. This work has been supported at Heliophysics, Inc., under NASA grants NNX09AP96G and NNX10AC09G.
The pressure to focus on math and reading at the elementary level has increased in recent years. As a result, science education has taken a back seat in elementary classrooms. The Think Scientifically book series provides a way for science to easily integrate with existing math and reading curriculum. This story-based science literature program integrates a classic storybook format with solar science concepts, to make an educational product that meets state literacy standards. Each story is accompanied by hands-on labs and activities that teachers can easily conduct in their classrooms with minimal training and materials, as well as math and language arts extensions. These books are being distributed through teacher workshops and conferences, and are available free at http://sdo.gsfc.nasa.gov/epo/educators/thinkscientifically.php.
A partnership between Stanford University and Chabot Community College (Hayward, CA) has developed a series of laboratory exercises using SDO (AIA, HMI) data, targeted for community college students in an introductory astronomy lab class. The labs lead students to explore what SDO can do via online resources and videos. Students investigate their chosen solar events, generate their own online videos, prepare their own hypotheses relating to the events, and explore outcomes. Final assessment should be completed by the end of summer 2013. Should the labs prove valuable, they may be adapted for high school use.
The Education and Public Outreach program for NASA's IRIS mission focuses on providing experiences in authentic solar science research to elementary and secondary students, STEM undergraduates, families and the public through science events, and public outreach through a variety of online resources. Our poster will highlight the achievements of these programs, details of their preparation, and use of collaborative partnerships to implement them.
EVE Education and Public Outreach (EPO) promotes an understanding of the process of science and concepts within solar science and sun-earth connections. EVE EPO also features working scientists, current research and career awareness. One of the highlights for of this years projects is the digitization of solar lessons and the collaboration with the other instrument teams to develop new resources for students and educators. Digital lesson suite: EVE EPO has taken the best solar lessons and reworked then to make then more engaging, to reflect SDO data and made them SMARTboard compatible. We are creating a website that Students and teachers can access these lesson and use them online or download them. Project team collaboration: The SDO instruments (EVE, AIA and HMI) teams have created a comic book series for upper elementary and middle school students with the SDO mascot Camilla. These comics may be printed or read on mobile devices. Many teachers are looking for resources to use with their students via the Ipad so our collaboration helps supply teachers with a great resource that teachers about solar concepts and helps dispel solar misconceptions.
The Eclipse Megamovie Project (EMP - www.eclipsemegamovie.org) is a multi-institutional collaboration designed to bring the public together to share their solar eclipse images and experiences and to become aware of and learn about the science of the Sun, Moon, and eclipses. We aim to do this by engaging the public through traditional avenues and via social media, and encouraging them via common technologies, e.g. iPads, iPhones, digital cameras etc, to share their total solar eclipse images at our project website. The first significant milestone for the EMP took place on the morning of November 14th 2012 as a total solar eclipse traversed Queensland, Australia. This eclipse provided a fantastic opportunity for public education and outreach about the Sun and its connection to our planet. With a very much smaller, controlled environment, this event also provided us with both a proof-of-concept, and a rare opportunity, to develop infrastructure and materials for a total solar eclipse that will transit the entire continental United States in August of 2017. The culmination of the EMP will take place on August 21st, 2017 as a total solar eclipse traverses the entire breadth of the continental United States, from Oregon to South Carolina. It will provide the opportunity to assemble a very large number of images, obtained by observers all along the path, into a continuous record of chromospheric and coronal evolution over that time - totality lasts for an hour and a half over the continental U.S. The 2017 exlipse will be a fantastic showcase for Sun-Earth connection, and engage as broad a swathe of the general public as possible. The EMP will outreach to K-12, colleges, and amateur astronomy groups (primarily in the states the eclipse passes over) via internet webcasts, social media, physical posters, and (where possible) visits by team members to foster interest, invovement and education. In this poster, we report on our experiences from the Queensland eclipse and our plans for the August 2017 eclipse and intervening eclipses.
The yearly National Student Solar Spectrograph Competition (NSSSC) is Montana Space Grant Consortium's Education and Public Outreach (EP/O) Program for NASA's Interface Region Imaging Spectrograph (IRIS) mission. The NSSSC is designed to give institutions with less aerospace activity such as Minority Serving Institutions and Community Colleges an opportunity for hands on real world research experience. The NSSSC provides students from across the country the opportunity to work as part of an undergraduate interdisciplinary team to design, build and test a ground based solar spectrograph. Over the course of nine months, teams come up with their own science goals and then build an instrument to collect data in support of their goals. Teams then travel to Bozeman, MT to demonstrate their instruments and present their results in a competitive science fair environment. This poster will present the 2012-2013 competition results.
Three of the most important and most puzzling features of the Sun’s atmosphere are the smoothness of the closed field corona (so-called coronal loops), the accumulation of magnetic shear at photospheric polarity inversion lines (filament channels), and the complex dynamics of the slow wind. We propose that a single process, helicity condensation, is the physical mechanism giving rise to all three features. A simplified model is presented for how helicity is injected and transported in the closed corona by magnetic reconnection. With this model we demonstrate that magnetic shear must accumulate at PILs and coronal hole boundaries, and estimate the rate of shear growth at PILs and the loss to the wind. Our results can account for many of the observed properties of the corona and wind. This work was supported in part by the NASA TR&T and SR&T Programs.
The Coronal Global Evolutionary Model, or CGEM, is a collaborative effort from the UC Berkeley Space Sciences Laboratory (SSL), Stanford University, and Lockheed-Martin. In work that led up to the selection of this project, the team demonstrated its capability to use sequences of vector magnetograms and Dopplergrams from the Helioseismic and Magnetic Imager (HMI) instrument aboard the SDO to drive a magnetofrictional (MF) model of the coronal magnetic field in AR 11158, which produced an X2.2 flare. We will implement this MF model in spherical coordinates to enable real-time, long-term modeling of the non-potential coronal magnetic field, both globally and for individual active region (ARs). The model's Earth-facing hemisphere will be driven using electric fields derived from the observed evolution of photospheric line-of-sight magnetic fields and electric currents. Far-side data inputs will be from an existing flux transport code, combined with HMI far-side observations of new active regions, with empirical parametrizations of orientation and flux. Because this model includes large-scale coronal electric currents, it is a substantial improvement over existing real-time global coronal models, which assume potential fields. Data products available from the model will include: 1) the evolving photospheric electric field, Poynting flux, and helicity flux; 2) estimates of coronal free energy and non-potential geometry and topology; 3) initial and time-dependent boundary conditions for MHD modeling of active regions; and 4) time-dependent boundary conditions and flux tube expansion factors for MHD and empirical solar wind models. Unstable configurations found from MF models will be dynamically evolved with local and global MHD codes. Modules used to derive surface electric fields from magnetic evolution will be incorporated into the SDO/HMI data pipeline, and data products will be distributed through the Joint Science Operations Center (JSOC) and directly to space weather forecasters and users. The electric field and MF codes will be delivered to the Community Coordinated Modeling Center (CCMC) for science analysis and use with other models. This project is being jointly funded by NASA and NSF.
Understanding coronal influences on the direction of propagation of CMEs is important for space weather prediction. It is well known that CMEs often propagate non-radially, e.g., they do not move out radially from the location of the solar source (see, e.g., Cremades and Bothmer, A&A, 2004; Panasenco et al., Sol. Phys. 2013). There is evidence that most CMEs exit the corona in the minimum field region surrounding the coronal/heliospheric current sheet (HCS). If this is the case, then the degree of deflection should reflect the distance of the source region from the current sheet region. Here we study the observed deflection in latitude of four CMEs using STEREO/SECCHI’s EUV and white light observations to trace the deflection. A potential-field source surface (PFSS) model (Schrijver & DeRosa, Sol. Phys. 2003) is used to give information on the magnetic forces acting on the CME at different heights in the lower corona. This model, as well as the PFSS model results at the GONG website (http://gong.nso.edu/data/magmap/archive.html) and the coronal observations from STEREO, are used to try to determine the location of the HCS. For the events studied, we find cases when the deflection is gradual (occurring between the surface at several solar radii) and cases where the deflection is immediate (within ~1.5 solar radii). There are many cases in the literature where CMEs originating at high latitude are deflected towards the ecliptic and eventually impact Earth. Several of the CMEs we analyzed were later detected in situ at ~1 AU and we compare the near-Sun trajectory information to the trajectory information determine from the in situ information.
Using metadata produced by automated solar feature detection modules developed for SDO (Martens et al. 2012) we have discovered some trends in filament chirality and filament-sigmoid relations that are new and in part contradict the current consensus. Automated detection of solar features has the advantage over manual detection of having the detection criteria applied consistently, and in being able to deal with enormous amounts of data, like the 1 Terabyte per day that SDO produces. Here we use the filament detection module developed by Bernasconi, which has metadata from 2000 on, and the sigmoid sniffer, which has been producing metadata from AIA 94 A images since October 2011. The most interesting result we find is that the hemispheric chirality preference for filaments (dextral in the north, and v.v.), studied in detail for a three year period by Pevtsov et al. (2003) seems to disappear during parts of the decline of cycle 23 and during the extended solar minimum that followed. Moreover the hemispheric chirality rule seems to be much less pronounced during the onset of cycle 24. For sigmoids we find the expected correlation between chirality and handedness (S or Z) shape but not as strong as expected.
Early in his productive career in astronomy, George Ellery Hale developed innovative instrumentation that allowed him to image the magnetically-dominated solar chromosphere. Among the solar phenomena he discovered were sunspot vortices, which he attributed to storms akin to cyclones in our own atmosphere. Much more recently, physicists discovered a quantity that is very well conserved in ideal magnetohydrodynamics: magnetic helicity. Our contemporary understanding of Hale's vortices as a consequence of large-scale twist in sunspot magnetic fields hinges on this conservation. I will review the crucial role that this property plays in the hemispheric and solar cycle dependences of Hales vortices, as well as solar flares and CMEs.
The structure of the coronal magnetic field prior to eruptive processes and the conditions for the onset of eruption are important issues that can be addressed through studying the magnetohydrodynamic stability and evolution of nonlinear force-free field (NLFFF) models. This talk uses data-constrained NLFFF models of a solar active region that erupted on 2010 April~8 as initial conditions in MHD simulations. These models, constructed with the techniques of flux rope insertion and magnetofrictional relaxation, include a stable, an approximately marginally stable, and an unstable configuration. The simulations confirm previous related results of magnetofrictional relaxation runs, in particular that stable flux rope equilibria represent key features of the observed pre-eruption coronal structure very well and that there is a limiting value of the axial flux in the rope for the existence of stable NLFFF equilibria. The specific limiting value is located within a tighter range, due to the sharper discrimination between stability and instability by the MHD description. The MHD treatment of the eruptive configuration yields very good agreement with a number of observed features like the strongly inclined initial rise path and the close temporal association between the coronal mass ejection and the onset of flare reconnection. Minor differences occur in the velocity of flare ribbon expansion and in the further evolution of the inclination; these can be eliminated through refined simulations. We suggest that the slingshot effect of horizontally bent flux in the source region of eruptions can contribute significantly to the inclination of the rise direction. Finally, we demonstrate that the onset criterion formulated in terms of a threshold value for the axial flux in the rope corresponds very well to the threshold of the torus instability in the considered active region.
Using 3D magneto-hydrodynamic simulations of the eruption of coronal flux ropes, we examine the thermal features produced by current sheet formation and the associated “tether-cutting” reconnections. We find that current sheets form along topological structures identified as quasi-separatrix layers (QSLs) during the pre-eruption stage. Tether-cutting reconnections in the current sheets produce a hot channel containing reconnected, twisted flux threading under the axis of the flux rope. This accumulation of twisted flux allows the flux rope to rise quasi-statically to the critical height for the onset of the torus instability, which leads to the dynamic eruption of the flux rope. The current sheet morphology and the hot channel that forms above it may explain the observed prominence “horns” enclosing a central cavity seen in AIA observations of coronal cavities. They may also be the cause of the X-ray emitting cores observed in some coronal cavities. We present a sequence of simulations to examine how the temperature and density of the hot channel depend on the properties of the coronal flux rope, and compare the results with multi-wavelength coronal observations of CMEs.
We present global coronal seismology, for the first time, that allows us to determine inhomogeneous magnetic field strengths in a wide range of the extended solar corona. We use observations of propagating disturbance associated with a coronal mass ejection observed on 2011 August 4 by the COR1 inner coronagraphs on board the STEREO spacecraft. We establish that the disturbance is in fact a fast magnetosonic wave as the upper coronal counterpart of the EIT wave observed by STEREO EUVI and travels across magnetic field lines with inhomogeneous speeds, passing through various coronal regions such as quiet/active corona, coronal holes, and streamers. We derive magnetic field strengths along the azimuthal trajectories of the fronts at heliocentric distances 2.0, 2.5, and 3.0 Rs, using the varying speeds and electron densities. The derived magnetic field strengths are consistent with values determined with a potential field source surface model and reported in previous works. The ranges of the magnetic field strengths at these heliocentric distances are 0.44 ± 0.29, 0.23 ± 0.15, and 0.26 ± 0.14 G, respectively. The uncertainty in determining magnetic field strengths is about 40 %. This work demonstrates that observations of fast magnetosonic waves by white-light coronagraphs can provide us with a unique way to diagnose magnetic field strength of an inhomogeneous medium in a wide spatial range of the extended solar corona.
From 54 X-ray jets observed in the polar coronal holes by Hinode’s X-Ray Telescope (XRT) during coverage in movies from Solar Dynamic Observatory’s Atmospheric Imaging Assembly (AIA) taken in its He II 304 Å band at a cadence of 12 s, we have established a basic characteristic of solar X-ray jets: untwisting motion in the spire. In this presentation, we show the progression of few of these X-ray jets in XRT images and track their untwisting in AIA He II images. From their structure displayed in their XRT movies, 19 jets were evidently standard jets made by interchange reconnection of the magnetic-arcade base with ambient open field, 32 were evidently blowout jets made by blowout eruption of the base arcade, and 3 were of ambiguous form. As was anticipated from the >10,000 km span of the base arcade in most polar X-ray jets and from the disparity of standard jets and blowout jets in their magnetic production, few of the standard X-ray jets (3 of 19) but nearly all of the blowout X-ray jets (29 of 32) carried enough cool (T ~ 10^5 K) plasma to be seen in their He II movies. In the 32 X-ray jets that showed a cool component, the He II movies show 10-100 km/s untwisting motions about the axis of the spire in all 3 standard jets and in 26 of the 29 blowout jets. Evidently, the open magnetic field in nearly all blowout X-ray jets and probably in most standard X-ray jets carries transient twist. This twist apparently relaxes by propagating out along the open field as a torsional wave. High-resolution spectrograms and Dopplergrams have shown that most Type-II spicules have torsional motions of 10-30 km/s. Our observation of similar torsional motion in X-ray jets (1) strengthens the case for Type-II spicules being made in the same way as X-ray jets, by blowout eruption of a twisted magnetic arcade in the spicule base and/or by interchange reconnection of the twisted base arcade with the ambient open field, and hence (2) strengthens the case made by Moore et al (2011, ApJ, 731: L18) that the Sun's granule-size emerging magnetic bipoles, by making Type-II spicules, power the global corona and solar wind. This work was funded by NASA’s LWS TRT Program, NASA's Hinode Project, and NSF's REU Program.
Recent studies suggest that the thermal energy input into a coronal mass ejection is comparable to the kinetic energy. The dissipation of magnetic energy is thought to be the source of this heating. One possible mechanism, the dissipation of Alfven waves, has generally been neglected because heating rates calculated from models of the fast solar wind are orders of magnitude less than what is required to match CME plasma observations. Using new a three-dimensional solar wind model driven by Alfven waves within the Space Weather Modeling Framework, we simulate eruptions in the low to middle corona. The goal is to explore the self-consistent heating of CME plasma by wave dissipation. We find that the expansion of a flux rope can create regions of enhanced plasma density at the back of the sheath, which we call piled-up compression (PUC) regions. The Alfven wave energy is also enhanced in the sheath, where surface Alfven wave damping due to the density gradients dissipates the wave energy. This heating rate is orders of magnitude larger than the heating rate in the fast solar wind, which suggests that Alfven wave dissipation may play a role in CME plasma heating.
We have finished digitizing all full disk and a fraction of high resolution films obtained by Big Bear Solar Observatory (BBSO) from 1969 to 1997. Using high-cadence (10s) digitized data we investigate a Moreton wave associated with an X9 flare on 1990 May 24, as well as its interactions with four filaments F1–F4 located close to the flaring active region. The interaction yields interesting insight into physical properties of both the wave and the filaments. The first Moreton wavefront appears at the active-region periphery at 21UT, about the same time as the peak of the microwave burst and the first of the double-peak gamma-ray burst. The wavefront propagates at 2 Mm/s within five minutes of its initiation, reaching as far as 600 Mm away from the flaring site. Sequential chromospheric brightenings (SCBs) are observed ahead of the Moreton wavefront, with similar appearance as the subsequent sequential brightenings due to the wave passage. A slower diffuse moving front at 300 o 600 km/s is observed to trail the fast Moreton wavefront about 1 min after the onset. The Moreton wave decelerates to 550 km/s as it sweeps through F1. The wave passage results in oscillations throughout the entire filament, predominantly perpendicular to F1’s spine, a temporary disappearance of F3 and F4 followed by a gradual recovery, but no disturbance in F2. Different height and magnetic configuration together may account for the distinct responses of the filaments to the wave passage. The wavefront bulges at F4 whose spine is oriented perpendicular to the upcoming wavefront. The deformation of the wavefront is suggested to be due both to the forward inclination of the wavefront and to the enhancement of the local Alfven speed within the filament channel.
We have observed a quiescent prominence with the Hinode Solar Optical Telescope (SOT, in Ca II and H-alpha lines), Sacramento Peak Observatory (in H-alpha, H-beta and Sodium-D lines), and THEMIS/MTR (Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires/MulTi Raies, providing vector magnetograms), and SDO/AIA (Solar Dynamics Observatory Atmospheric Imaging Assembly, in EUV) over a 4 hour period on 2012 October 10. The small fields of view of SOT, Sac Peak and THEMIS are centered on a large pillar-like prominence footpoint extending towards the surface. This feature appears in the larger field of view of the 304 Å band, as a large, quasi-vertical column with material flowing horizontally on each side. The THEMIS/MTR data indicate that the magnetic field in the pillar is essentially horizontal and the observations in the optical wavelengths show a large number of horizontally aligned features on a much smaller scale than the pillar as a whole. The data are consistent with a model of cool prominence plasma trapped in the dips of horizontal field lines. The SOT and Sac Peak data show what appear to be moving wave pulses. These pulses, which include a Doppler signature, move vertically, perpendicular to the field direction, along quasi-vertical columns. The pulses have a velocity of propagation of about 10 km/s, a period about 260 sec, and a wavelength around 2000 km. We interpret these waves in terms of fast magneto-sonic waves and discuss possible wave drivers.
Recently, ubiquitous coronal loop oscillations were detected in active region loops by SDO/AIA. Hinode/EIS observations indicate that quasi-periodic flows are present at footpoints of loops in active regions, and related propagating disturbances (PD's) were detected in open and closed loop structures. Recent 3D MHD models in idealized (bipolar) active regions (Ofman et al. 2012; Wang et al. 2013, this meeting) have demonstrated that the flows can produce slow magnetosonic waves in loops, as well as transverse oscillations. We extend the idealized studies by considering more realistic magnetic field structures modeled by including photospheric magnetic field extrapolated to the corona as boundary and initial conditions for the 3D MHD modeling. We use potential and nonlinear magnetic field extrapolations combined with gravitationally stratified density and introduce flows at the corona-transition region boundary in our 3D MHD model. We apply coronal seismology to the resulting loop oscillations and compare to oscillation events detected by SDO/AIA. We aim to improve the accuracy of coronal seismology by modeling coronal loop oscillations in realistic magnetic geometry and density structures.
We report results the analysis of coronal fan loops in a non-flaring solar active region exhibiting temperature-dependent propagating optical disturbances. A 6-hour set of high resolution coronal observations provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) has been used for characterizing apparent propagating patterns at multiple coronal temperatures (131A, 171A, 193A and 211A). A new data analysis methodology has been developed enabling an identification of subvisual motions with low signal-to-noise ratios not previously examined in this context. The technique involves spatiotemporal tracking of fan loop filaments containing propagating disturbances, construction of position - time plots for different temperature channels, obtaining the waveforms of the propagating optical features, and evaluation of Fourier spectral power of the waveforms as a function of phase speed and frequency. Using this methodology, we identified the parameters of propagating optical disturbances in different magnetic geometries, and classified these events as waves and/or plasma jets. We explored coronal conditions favoring wave-like and jet-like traveling plasma density enhancements in fan loops and the mechanisms of their generation, damping and interaction. The results obtained are compared with the behavior of a resistive MHD model exhibiting both types of propagating disturbances.
The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket launched 23 April 2013 at 17:30 UT, as part of a campaign including co-ordinated observations with the Dunn Solar Telescope/IBIS, Hinode/EIS, SoHO/CDS, RHESSI and SDO. EUNIS obtained the highest-resolution observations of the solar spectrum from 52-63 nm observed to date, as well as observations with the previously-flown waveband from 30-37 nm. The broad spectral coverage of the EUV observations includes emission lines of ionization stages from He I to Fe XIX, and thus a wide temperature range of 0.025 to 10 MK. Absolute radiometric calibration of EUNIS provides underflight calibration of CDS, EIS and AIA. Spectra were obtained with a 1.3 s cadence as the 660-arcsec long slit was rastered across two different regions. The observations captured a B-class flare in active region NOAA 11726 as well as active regions 11723, 11724, off-limb, quiet sun and a coronal hole. We discuss first results from anaysis of this rich and extensive data set.
Chromospheric flares have been carefully observed and studied for many decades. Ribbons of hot plasma appear, brighten, and separate during the course of a flare. Adjacent to eruptions with associated coronal mass ejections, compact brightenings are observed in the impulsive phase of the flare. What causes these compact brightenings adjacent to flares? What can they tell us about the solar conditions that formed the chromospheric flare? We present an automated algorithm to identify, track, and characterize small-scale brightening associated with solar eruptive phenomena observed in H-alpha. The temporal, spatially localized changes in chromospheric intensities can be separated into two categories: flare ribbons and sequential chromospheric brightenings (SCBs). We then report on the physical properties of SCBs. Following the algorithmic identification and a statistical analysis, we compare and find the following: SCBs are distinctly different from flare brightening in their temporal characteristics of intensity, Doppler structure, duration, and location properties. Within the studied population of SCBs, different classes of characteristics are observed with coincident negative, positive, or both negative and positive Doppler shifts of a few km/s. The appearance of SCBs often precedes peak flare intensity. They are also found to propagate laterally away from flare center in clusters at two distinct velocity groups. Given SCBs’ distinctive nature compared to flares, we suggest a physical triggering mechanism relating to SCBs’ origin, the associated flare, and coronal mass ejections. We present a heuristic model of SCBs in the chromosphere.
Filament channels coincide with large-scale polarity inversion lines of the photospheric magnetic field, where flux cancellation continually takes place. High-cadence Solar Dynamics Observatory (SDO) images recorded in He II 30.4 nm and Fe IX 17.1 nm in August 2010 reveal numerous transient brightenings occurring along the edge of a filament channel within a decaying active region, where SDO line-of-sight magnetograms show strong opposite-polarity flux in close contact. The brightenings are elongated along the direction of the filament channel, with linear extents of several arcseconds, and typically last a few minutes; they sometimes have the form of multiple two-sided ejections with speeds on the order of 100 km/s. Remarkably, some of the brightenings rapidly develop into larger scale events, forming sheetlike structures that are eventually torn apart by the diverging flows in the filament channel and ejected in opposite directions. In some cases, the flow patterns that develop in the channel may bring successive horizontal loops together and cause a cascade to larger scales. One of these brightening events was the initiation of a large-amplitude longitudinal oscillation of the filament. We interpret the brightenings as resulting from reconnections among filament-channel field lines having one footpoint located in the region of canceling flux.
There are two parts in my presentation. In the first part I present the magnetic field measurement of an active region filament using the full Stokes profiles of He I 10830 and Si I 10827 band when the filament in its stable phase. This observation was fulfilled using German Vacuum Tower Telescope (VTT). The vector magnetic field and Doppler velocity map both in the photosphere and chromosphere were observed and analyzed co-temporally and co-spatially. The observation findings reveal that we were observing the emergence of a flux rope with a subsequent formation of a filament. In the second part, I would like to exhibit another ground-based observation facility, 1m New Vacuum Solar Telescope (NVST) located in Fu-Xian Lake Solar Observatory of China. After the basic introduction including the location and instrumentations, I give some high lights including granulation, faculae, micro-flares, jets, and filaments or prominence since the first running in 2010, showing our potential ability to do high-resolution solar observation from the ground. Observation proposals from the international solar community are well appreciated in future.
The Hi-C sounding rocket data can be used to determine the rate and approximate magnitude of free energy released by the active region coronal magnetic field. We present the results from the analysis of two regions within the Hi-C field-of-view. Hi-C data is also used to determine the temperature and density of structures and we offer conclusions on the spatial scale for these features.
We explore the process by which a coronal loop can become heated in response to an ideal magnetic field instability. A three-dimensional magnetohydrodynamic Lagrangian-remap code is used to simulate the evolution of a specific line-tied field configuration, which is based on a zero-net-current cylindrical loop model. The initial loop state is known to be linearly kink unstable. In addition, the field surrounding the loop is potential (the external field is parallel to the initial loop axis). The kink instability rapidly leads to the formation of slow mode shocks within the loop interior, where the pressure and sound speed are low compared to the loop boundary. We investigate how these shocks influence the heating process. In general, slow mode shocks act to release magnetic energy in the form of currents, which then give rise to steep velocity gradients at the loop boundary. It is this last feature that causes shock heating, represented in the code as artificial viscosity. Our model also incorporates thermal conduction, radiation and gravity; thus, we can forward model our results to show the appearance of the loop within the 171 ? passband used by the AIA instrument onboard the Solar Dynamics Observatory.
High-resolution UV and EUV observations have revealed that flare loops are formed and heated by reconnection events taking place successively. Our recent work (Qiu et al. 2012) suggests that the rapid rise of UV brightness at the foot-points of individual flare loops could be used to infer the impulsive heating rate in these loops. Using these heating rates and the Enthalpy-Based Thermal Evolution of Loops (EBTEL, Klimchuk et al. 2008, Cargrill et al. 2012) model, we can compute plasma evolution in thousands of flare loops anchored at the UV foot-points, and calculate the synthetic coronal radiation by these loops to compare with observations. Therefore, the method uses observations to constrain the heating rates from both the input and output of the loop heating model. In this study, we apply this method to two M-class flares occurred on 2005 May 13 and 2011 March 07, respectively, and show that the synthetic soft X-ray and EUV spectra and light curves compare favorably with the observations by RHESSI and EVE. With a steady-state assumption, we also compute the transition-region DEM at the base of each flare loop during its decay phase, and compare the predicted UV and EUV emission at the foot-points with AIA observations. This experiment provides another independent constraint to determination of the heating rates. Furthermore, using RHESSI hard X-ray observations, we also infer the fraction of non-thermal beam heating in the total heating rate of flare loops, and discuss its effect on plasma evolution. For the 2005 May 13 M8.0 flare that exhibits significant thick-target hard X-ray emissions, the lower limit of the total energy used to heat the flare loops is $1.2 \times 10^31$ ergs, out of which, less than 20% is carried by beam-driven upflows during the impulsive phase.
Understanding electron acceleration in solar flares requires X-ray studies with greater sensitivity and dynamic range than are available with current solar hard X-ray observers (i.e. the RHESSI spacecraft). RHESSI employs an indirect Fourier imaging method that is intrinsically limited in dynamic range and therefore can rarely image faint coronal flare sources in the presence of bright footpoints. With greater sensitivity and dynamic range, electron acceleration sites in the corona could be studied in great detail. Both these capabilities can be advanced by the use of direct focusing optics. The recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload demonstrates the feasibility and usefulness of hard X-ray focusing optics for solar study. FOXSI features grazing-incidence replicated nickel optics from the NASA Marshall Space Flight Center and fine-pitch silicon strip detectors developed by the Astro-H team at JAXA/ISAS. FOXSI flew successfully on November 2, 2012, producing images and spectra of a microflare and performing a search for nonthermal emission (4-15 keV) from nanoflares in the quiet Sun. Nanoflares are a candidate for providing the required energy to heat the solar corona to its high temperature of a few million degrees. A future satellite version of FOXSI, featuring similar optics and detectors, could make detailed observations of hard X-rays from flare-accelerated electrons, identifying and characterizing particle acceleration sites and mapping out paths of energetic electrons as they leave these sites and propagate throughout the solar corona.
We focus on investigating heating and evolution of flare loops. A C4.7 two-ribbon flare on 2011 February 13 is analyzed and modeled. From AIA imaging observations, we can identify two sets of loops in this event. EIS spectroscopic observations reveal blueshifts at the feet of both sets of loops during the impulsive phase. However, the dynamic evolutions of the two sets of loops are quite different. The first set of loops exhibits blueshifts (~10 km/s) for about 25 minutes followed by redshifts, while the second set shows stronger blueshifts (~20 km/s) which are maintained for about 1 hour. The long-lasting blueshifts in the second set of loops are indicative of continuous heating. The UV 1600 Å observation by AIA also shows that the feet of the loops brighten twice with 15 minutes apart. The first set of loops, on the other hand, brighten only once in the UV band. We construct heating functions of the two sets of loops using spatially resolved UV light curves at their footpoints, and model plasma evolution in these loops with the EBTEL model. The results show that, for the first set of loops, the synthetic EUV light curves from the model compare favorably with the observed light curves in six AIA channels and eight EIS spectral lines, and the computed mean enthalpy flow velocities also agree with the Doppler shifts measured in EIS lines. For the second set of loops modeled with twice-heating, there are some discrepancies between modeled and observed EUV light curves at low-temperature lines, and the model does not fully reproduce the prolonged blueshift signatures as observed. The prominent discrepancies between model and observations for the second set of loops may be caused by non-uniform heating localized especially at the loop footpoints which cannot be modeled by the 0D EBTEL model, or by unresolved fine flaring strands in the loops with quite different heating rates and profiles.
The coupling between the chromosphere and corona is a question of great current interest. It has long been understood that coronal mass originates in the chromosphere and that the energy which powers the corona flows up through the chromosphere. However, the details of how this happens are now being questioned. In the traditional view, “mechanical” energy flows into the corona in the form of waves or gradually increasing magnetic stresses. The waves and stresses dissipate and heat the plasma. The resulting downward thermal conduction flux causes material to evaporate from the chromosphere and fill the corona. If the heating is steady, an equilibrium is established whereby radiation and thermal conduction balance the energy input. If the heating is impulsive (a nanoflare), the evaporated plasma cools and drains, only to reappear during the next event. In either case, the heating occurs in the corona. A new idea is that the heating occurs instead in the chromosphere. Cold plasma is directly heated to coronal temperatures and then flows upward due to expansion and perhaps also an ejection process. The hot tips of type II spicules are one example, though spicules need not be involved. I will discuss these two fundamentally different scenarios and the observational predictions that they make. A comparison with actual observations leads to the conclusion that only a small fraction of the hot plasma in the corona comes from chromospheric heating. Most coronal plasma is a consequence of heating that occurs in the corona itself.
The spatial and spectral properties of three solar flare coronal X-ray loops are studied before, during, and after the peak X-ray emission. Using observations from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we deduce the temporal changes in emitting X-ray length, corpulence, volume, position, number density, and thermal pressure. We observe a decrease in the loop length, width, and volume before the X-ray peak, and an increasing number density and thermal pressure. After the X-ray peak, volume increases and loop corpulence grows due to increasing width. The volume variations are more pronounced than the position variations, often known as magnetic field line contraction. We believe this is the first dedicated study examining the temporal evolution of X-ray loop lengths and widths. Collectively, the observations also show for the first time three temporal phases given by peaks in temperature, X-ray emission, and thermal pressure, with the minimum volume coinciding with the X-ray peak. Although the volume of the flaring plasma decreases before the peak in X-ray emission, the relationship between temperature and volume does not support simple compressive heating in a collapsing magnetic trap model. Within a low ? plasma, shrinking loop widths perpendicular to the guiding field can be explained by squeezing the magnetic field threading the region. Plasma heating leads to chromospheric evaporation and growing number density. This produces increasing thermal pressure and decreasing loop lengths as electrons interact at shorter distances and we believe after the X-ray peak, the increasing loop corpulence.
Magnetic Flux Ropes Immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Each collision results in magnetic field line generation and the generation of a quasi-seperatrix layer. Three dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. When the currents associated with the ropes are large,this is possible for only a number of rotation cycles as the field line motion becomes chaotic. The permutation entropy1 can be calculated from the the time series of the magnetic field data (this is also done with flows) and used to calculate the positions of the data on a Jensen Shannon complexity map2. The power spectra of much of the magnetic and flow data is exponential and Lorentzian structures in the time domain are embedded in them. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The complexity map, and analysis will be explained in the course of the talk. Other types of chaotic dynamical models such as the Lorentz or Gissinger process also fall on the map and can give a clue to the nature of the flux rope turbulence. The ropes fall in the region of the C-H plane where chaotic systems lie. 1 C. Bandt, B. Pompe, Phys. Rev. Lett., 88,174102 (2007) 2 O. Russo et al., Phys. Rev. Lett., 99, 154102 (2007), J. Maggs, G.Morales, “Permutation Entropy analysis of temperature fluctuations from a basic electron heat transport experiment”,submitted PPCF (2013)
The fan-spine magnetic topology is believed to be responsible for many curious emission signatures in solar explosive events. A spine field line links topologically distinct flux domains and possibly their evolutionary trends, but direct observation of such structure has been rare. Here we report a unique event observed by the Solar Dynamic Observatory (SDO) where a set of hot coronal loops (over 10 MK) that developed during the rising phase of the flare connected to a quasi-circular chromospheric ribbon at one end and a remote brightening at the other. Magnetic field extrapolation suggests these loops are partly tracers of the evolving spine field line. The sequential brightening of the ribbon, the apparent shuffling loop motion, and the increasing volume occupied by the hot loops suggest that continuous slipping- and null-point-type reconnections were at work, energizing the loop plasma and transferring magnetic flux within and across the dome-shaped, fan quasi-separatrix layer (QSL). We argue that the initial reconnection is of the 'break-out' type, which transitioned to more violent flare reconnection nearing the flare peak with an eruption from the fan dome. Significant magnetic field changes are expected and indeed ensued, which include a shift of the QSL footprint, an increase of the horizontal photospheric field, and de-shearing of the coronal loops. This event also features an extreme-ultraviolet (EUV) late phase -- a second emission peak observed in the warm EUV lines (about 2--7 MK) up to 1--2 hours after the soft X-ray peak. We show that this peak comes from the large post-flare arcades beside and above the compact fan dome, a direct product of eruption in such topological settings. Cooling of these large arcades naturally explains the sequential delay of the late-phase peaks in increasingly cooler EUV lines, and the estimated theoretical cooling time is compatible with observation. Our result demonstrates the critical nature of cross-scale magnetic coupling -- minor topological change in a sub-system may lead to explosions on a much larger scale.
We developed a forward-fitting code that computes a nonlinear force-free magnetic field (NLFFF) solution constrained by line-of-sight magnetograms from HMI/SDO and by coronal loop structures detected in EUV images from AIA/SDO. The 2D coordinates of coronal loop structures are detected with an improved version of the Oriented Coronal CUrved Loop Tracing (OCCULT-2) code, an automated pattern recognition algorithm that has demonstrated a quality and fidelity in loop tracing that matches visual perception. One fundamental limitation in the completeness of detecting coronal loops comes from the background confusion of coronal loop EUV emission with low-temperature (T=10^4-10^6 K) emission from the chromosphere and transition region, as well as T ~ 1.0 MK emission from reticulated ``moss structure'' that stems from the footpoints of hotter (T `~ 2-8 MK) coronal loops. We employ a pixel-wise differential emission measure (DEM) analysis using the 7 coronal AIA filters in order to produce uncontaminated emission measure maps in coronal temperature ranges, which allows an improved performance of automated loop tracing. A nonlinear force-free magnetic field solution is then computed by forward-fitting of an analytical NLFFF solution of twisted coronal field lines to the automatically traced coronal loop coordinates. We demonstrate the performance of this magnetic field modeling for a number of solar active regions observed with SDO. The developed method is able to calculate the most realistic magnetic field models of solar active regions that match all available observable constraints.
Coronal loops have been observed for several decades, yet some of their properties remain a mystery. These in particular include the lack of apparent expansion of coronal loops and the increased pressure scale height in loops compared to the diffuse background. We approach these problems in an entirely new way. We demonstrate that solely lifting the assumption about circular cross-sectional shape of flux tubes is alone sufficient to explain lack of expansion and increased pressure scale height. While magnetic flux tubes expand in the corona, they do so in a highly anisotropic manner, which we examine in details for several model fields and quantify for a potential field model based on HMI data. We demonstrate how, and why, this leads towards (1) selection bias which might make some loops stand out if they expand mostly along the line of sight, due to their increased column depth; (2) principal limitations on measuring expansion of coronal loops, even if they are resolved and (3) the apparent increased pressure scale height. We also address the existing studies which seemingly concluded the opposite. The latter was based on several properties of the loops' emission which, as we show, are also reproduced when loops are oblate in cross-section.
We show detailed results of a combined DEM and density-sensitive line ratio analysis of coronal loops observed simultaneously by EIS and AIA. The temperature and density profiles of the loop are compared to and isolated from those of the surrounding material, and these properties are fit to an analytic strand heating model developed by Martens (2010). This research builds on our previously reported work by analyzing a number of coronal loops (including one observed by the Hi-C rocket), improved background subtraction and loop fitting. These improvements allow us to place significant constraints on the heating distribution of coronal loops.
New opportunities for suborbital research are on the horizon. Reusable suborbital vehicles will offer immediate and routine space access for scientific payloads, provide access to altitudes around 100 kilometers, create opportunities for low-cost monitoring of upper atmospheric phenomena, as well as small scale solar observation. Reduced operational cost and quick turn-around will enable equipment to be flown opportunistically, in response to specific solar activity, or in continuous test and improvement cycles. Suborbital test flights will also provide opportunities to test prospective satellite instruments in an extended microgravity environment before being launched to orbit, raising the technology readiness level (TRL) of flight hardware and reducing the risk of anomalies during missions. I discuss the capabilities of emerging suborbital vehicles, payload and integration requirements, and funding opportunities for suborbital flights at NASA.
Flux-rope-based models of solar eruptions rely on the formation of a line-tied flux rope equilibrium that persists until an ideal instability or a breakdown in force balance triggers an eruption. In this paper, we present a quantitative study of equilibrium force balance in solar-relevant flux ropes, focusing primarily on the role of the potential magnetic field in controlling the flux rope behavior. This study was conducted using a newly constructed laboratory experiment in conjunction with supporting three-dimensional MHD simulations that directly model the experimental geometry. The flux ropes studied here, which are produced in the Magnetic Reconnection Experiment (MRX), evolve quasi-statically over many Alfvén times and have footpoints that are line-tied to two fixed electrodes [E. Oz, C. E. Myers, M. Yamada, et al., Phys. Plasmas 18, 102107 (2011)]. They are formed within a solar-relevant potential magnetic field configuration that can be systematically modified between discharges. Detailed in situ magnetic measurements from the experiments are compared directly to results from the simulations in order to quantitatively evaluate the various contributions to the equilibrium force balance. We find that forces derived from the applied toroidal guide field contribute significantly to the equilibrium—so much so that the flux ropes are often well confined even in the absence of a 'strapping' arcade. These observed guide field forces arise from changes in the toroidal magnetic pressure and tension that result from a combination of effects within the expanding flux rope. With regard to eruptions, the aforementioned guide field forces supplement the well-known strapping field forces to largely prevent the flux ropes from erupting. In particular, many regimes were explored where the strapping field configuration is predicted to be 'torus unstable' and yet the flux ropes do not erupt. Eruptions are observed in some regimes, however, and we will discuss the physical mechanisms that may explain this behavior.
We will present preliminary results on the investigation of one polar crown prominence that erupted on 2012 March 12. This prominence is viewed at the east limb by SDO/AIA and displays a simple vertical-thread structure. Bright U-shape (horn-like) structure is observed surrounding the upper portion of the prominence before the eruption and becomes more prominent during the eruption. When viewed on the disk, STEREO-B shows that this prominence is composed of series of vertical threads and displays a loop-like structure during the eruption. We focus on the magnetic support of the prominence by studying the structure and dynamics before and during the eruption using observations from SDO, Hinode, and STEREO. We will explore magnetic field modeling of this prominence using the flux rope insertion method. We will also present preliminary analysis on the thermodynamics of the prominence, namely DEM analysis of the cavity surrounding the prominence, as well as column density measurements. This work is supported by NASA Grant (#NNX12AB25G) and NASA Contract (#SP02H1701R) from LMSAL to SAO.
The effect of the numerical spatial resolution in models of the solar corona and corona / chromosphere interface is examined for impulsive heating over a range of magnitudes. It is demonstrated that the principle effect of inadequate resolution is on the coronal density. An under-resolved loop typically has a peak density of at least a factor of two lower than a resolved loop subject to the same heating, with larger discrepancies in the decay phase. The temperature for under-resolved loops is also lower indicating that lack of resolution does not “bottle up” the heat flux in the corona. Energy is conserved in the models to under 1% in all cases, indicating that this is not responsible for the low density. Instead, we argue that in under-resolved loops the heat flux “jumps across” the transition region to the dense chromosphere from which it is radiated rather than heating and ablating transition region plasma. This emphasizes the point that the interaction between corona and chromosphere occurs only through the medium of the transition region.
Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar and heliospheric activity; therefore, understanding the structure and dynamics of the coronal magnetic field is central to understanding this activity. Important heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions where solar eruptions occur agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does. We present a 3D model that explains how flare-accelerated particles escape onto interplanetary magnetic flux tubes during a solar eruption. Our model is based on results from large-scale 3D MHD simulations of a breakout-CME erupting into a heliosphere with an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnection episodes that occur during the evolution of the event, and show how they lead to the release of flare-accelerated particles onto open field lines. Analyzing the dynamics of the reconnected flux during the eruption, we evaluate the spatial distribution and the timing of the particle beams injected into the heliosphere. We discuss the implications of results for CME/flare models and for SEPs observations. This work was supported, in part, by the NASA TR&T and SR&T Programs.
We analyze spectropolarimetric data of a limb active region (NOAA 11302) obtained on September 22nd 2011 using the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope (DST). Stokes profiles including lines of Si I 1028.7 nm and He I 1083 nm were obtained in three scans over a 45'x75' area. Simultaneous narrow band Ca II K and G-band intensity data were acquired with a cadence of 5s at the DST. The He I data show not only typical active region polarization signatures, but also signatures in plumes -- cool post flare loops -- which extend many Mm into the corona across the visible limb. The plumes have remarkably uniform brightness, and the plume plasma is significantly Doppler shifted as it drains from the corona. Using carefully constructed observing and calibration sequences and applying Principal Component Analysis to remove instrumental artifacts, we achieved a polarization sensitivity approaching 0.02%. With this sensitivity we attempt to diagnose the vector magnetic fields and plasma properties of chromospheric and cool coronal material in and above NOAA 11302. Inversions using various radiative transfer models in the HAZEL code are remarkably consistent with the idea that plume spectra are formed in a simple, slab-like geometry, but that the ``disk'' spectra are formed under more traditional models (Milne-Eddington). The inverted magnetic data of He I lines are compared with photospheric inversions of DST Si I and Fe I data from the Solar Dynamics Observatory.
Space based measurements of sunspots are still relatively new when compared with the long visual record that is available from ground based observatories, but they can provide many advantages that ground based measurements cannot. Sunspots are automatically detected from SOHO/MDI and SDO/HMI continuum images using the Sunspot Tracking And Recognition Algorithm (STARA). A self consistent sunspot catalogue is created using the same criteria for detecting sunspots throughout time, eliminating effects seen by ground observatories such as changing observers or the effect of an observers eyesight as they age. This catalogue is then analysed to determine how sunspots evolve and what their population is in a number of physical parameters, which is of great importance for simulations of magnetic flux emergence and the solar dynamo. In particular, the change in sunspot parameters between solar cycles 23 and 24 is of great interest as the cycles appear vastly different in activity, and sunspots are a primary indicator of the activity of the Sun. The catalogue is also freely available for use by the community.
Since the launch of Solar Dynamics Observatory, the Helioseismic and Magnetic Imager has accumulated 3 years of continuous observations. Using time-distance helioseismology, we have obtained new results on both global and local scales. By analyzing the first two years' observations, we were able to detect the equatorward meridional flow at a depth of around 65 Mm, and detect the existence of a second meridional circulation cell below about 120 Mm. This new profile of interior meridional flow will pose challenges to the solar dynamo models. At the shallower depths, we studied the temporal evolution of the zonal and meridional flows. We found that both quantities showed strong hemispherical asymmetries. Using global wavefield simulations that have pre-set meridional flow profiles, we also assess the capability of our analysis technique in recovering week flows in the deep interior.
Using a thin flux tube model in a rotating spherical shell of turbulent, solar-like convection, we find that the distribution of emerging flux tubes in our simulation is inhomogeneous in longitude, with properties similar to those of active longitudes on the Sun and other solar-like stars. The large-scale pattern of flux emergence our simulations produce exhibit preferred longitudinal modes of low order, drift with respect to a fixed reference system, and show alignment at low latitudes within 15 degrees on either side of the equator. We suggest that these active-longitude-like emergence patterns are the result of columnar, rotationally aligned giant cells present in our convection simulation at low latitudes. If giant convecting cells exist in the bulk of the solar convection zone, this phenomenon, along with differential rotation, could in part provide an explanation for the existence and behavior of active longitudes.
Coronal mass ejections (CMEs) and solar flares are closely related in various ways because the two phenomena are different manifestations of the same energy release in closed magnetic regions on the sun. Of particular interest is the relation between flare reconnection flux at the Sun and the poloidal flux of the 1-AU flux rope associated with the flare. If a flux rope forms due to flare reconnection, then the two fluxes are almost equal. The flare reconnection flux is normally computed from the flare-ribbon area and the photospheric field strength in the ribbon area. Here we report on another technique, which makes use of the area under the post-eruption arcade (PEA). We show that the reconnection flux derived from the PEA technique agrees with the one derived from flare ribbons. We also fit a flux rope to the white-light CME observations and derive the aspect ratio of the flux rope. Assuming self-similar expansion of the flux rope, we show that the magnetic content and size of the 1-AU flux rope can be predicted from the flare magnetic field (the average photospheric field strength within half of the PEA area) and the aspect ratio of the coronal flux rope. We illustrate the method with several examples.
Identifying and understanding (1) the coronal heating mechanism and (2) the acceleration mechanism for the high-speed solar wind are two of the most important modern problems in solar physics. Many competing models of the high-speed solar wind depend on the solar magnetic field inside heliocentric distances of 5 solar radii. We report on sensitive VLA full-polarization observations made in August, 2011, at 5.0 and 6.1 GHz (each with a bandwidth of 128 MHz) of the radio galaxy 3C228 through the solar corona at heliocentric distances of 4.6 – 5.0 solar radii. Observations at 5.0 GHz (C-band frequencies) permit measurements deeper in the corona than previous VLA observations at 1.4 and 1.7 GHz. These Faraday rotation observations provide unique information on the plasma density and magnetic field strength in this region of the corona. The measured Faraday rotation on this day was lower than our a priori expectations, but we have successfully modeled the measurement in terms of observed properties of the corona on the day of observation. Further, 3C228 provides two lines of sight (separated by 46”) that allow measurement of differential Faraday rotation. These data may provide constraints on the magnitude of coronal currents and, thus, on the role Joule heating plays in the corona. Fluctuations in the observed rotation measure may also place constraints on wave-turbulence models by constraining the magnitude of coronal Alfvén waves.
An investigation of helicity injection by photospheric shear motions is carried out for two active regions (ARs), NOAA 11158 and 11166, using line-of-sight magnetic field observations obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We derived the horizontal velocities in the ARs from the differential affine velocity estimator (DAVE) technique. Persistent strong shear motions at maximum velocities in the range of 0.6–0.9 km s?1 along the magnetic polarity inversion line and outward flows from the peripheral regions of the sunspots were observed in the two ARs. The helicities injected in NOAA 11158 and 11166 during their six-day evolution period were estimated as 14.16x10$^42$ Mx$^2$ and 9.5x10$^42$ Mx$^2$, respectively. The estimated injection rates decreased up to 13% by increasing the time interval between the magnetograms from 12 minutes to 36 minutes, and increased up to 9% by decreasing the DAVE window size from 21x18 to 9x6 pixel$^2$, resulting in 10% variation in the accumulated helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous helicity flux distribution with mixed helicities of both signs and coronal mass ejection (CME) prone regions had almost homogeneous distribution of helicity flux dominated by a single sign. The temporal profiles of helicity injection showed impulsive variations during some flares/CMEs due to negative helicity injection into the dominant region of positive helicity flux. A quantitative analysis reveals a marginally significant association of helicity flux with CMEs but not flares in AR 11158, while for the AR 11166, we find a marginally significant association of helicity flux with flares but not CMEs, providing evidence of the role of helicity injection at localized sites of the events. These short-term variations of helicity flux are further discussed in view of possible flare-related effects. This study suggests that flux motions and spatial distribution of helicity injection are important to understanding the complex nature of the magnetic flux system of the AR, and how it can lead to conditions favorable for eruptive events.
The formation, evolution and eruption of solar active regions are among the main themes of research in solar physics. Special kinds of S-shaped active regions (sigmoids) facilitate this line of research, since they provide conditions that are easier to disentangle and have been shown to possess high probability for erupting as flares and/or coronal mass ejections (CME). Several theories have been proposed for the formation, evolution, and eruption of solar active regions. Testing these against detailed models of sigmoidal regions can provide insight into the dominant mechanisms and conditions required for eruption. We explore the behavior of solar sigmoids via both observational and magnetic modeling studies. Data from the most modern space-based solar observatories are utilized in addition to state-of-the-art, three-dimensional, data-driven magnetic field modeling to gain insight into the physical processes controlling the evolution and eruption of solar sigmoids. We use X-ray observations and the magnetic models to introduce the underlying magnetic and plasma structure defining these regions. By means of a large, comprehensive observational study, we look at the formation and evolution mechanism. Specifically, we apply additional analysis to show that flux cancellation is a major mechanism for building the underlying magnetic structure associated with sigmoids, namely magnetic flux ropes. We make use of topological analysis to describe the complicated magnetic field structure of the sigmoids. We show that when data-driven models are used in sync with MHD simulations and observations, we can arrive at a consistent picture of the scenario for CME onset, namely the positive feedback between reconnection at a generalized X-line and the torus instability.
Solar flares and coronal mass ejections involve massive releases of energies into the heliosphere and are the main driver of space weather disturbances near Earth. It is now well accepted that these enigmatic events are manifestations of a sudden and violent disruption of the Sun's coronal magnetic field. However, although such eruptions have been studied for many years, the detailed physical mechanisms by which they are initiated and driven are not yet fully understood; primarily because of our present inability to accurately measure magnetic fields in the corona. Numerical models have become a powerful tool to help us overcome this limitation. Global simulations of solar eruptions are particularly challenging, because of the enormous disparity of the relevant scales. While the steady advance of computational power has enabled us to model eruptions with ever increasing detail and realism, many questions remain unanswered. In this talk, I review what we have learned from numerical modeling about the physical processes associated with solar eruptions and I will discuss the current limitations and future prospects of models.
The magnetic carpet is defined to be the small-scale photospheric magnetic field of the quiet-Sun. Recent high resolution, high cadence observations have shown that although small-scale, the magnetic carpet is far from 'quiet', it is continually evolving in a complex and dynamic manner. I will present a two-component model for the dynamic evolution of the Sun's magnetic carpet. The first component is a 2D model for the photospheric evolution of the small-scale solar magnetic field, that reproduces many observed parameters. The basic evolution of magnetic elements within the model is governed by a supergranular flow profile. In addition, magnetic elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. The synthetic magnetograms produced by the 2D model are then applied as photospheric boundary data to drive the continuous evolution of a 3D non-linear force-free coronal field. We studied the resultant complex, small-scale coronal magnetic field, in particular the energetics of the field.
In NOAA Active Region 11112, a small bipole emerges into an area of preexisting, unipolar flux. The bright, low lying kernel of coronal loops above the emerging field, observed with AIA and XRT, originally show magnetic connectivity only between regions of newly emerged flux when overlaid on HMI magnetograms. Over the course of several days, this bright kernel advances into the preexisting flux. The advancement of this easily visible boundary into the old flux regions over time provides a quantifiable rate of reconnection between old and new magnetic domains. We compare the reconnection rate to the inferred heating of the coronal plasma. To our knowledge, this is the first measurement of steady, quiescent heating related to reconnection. While AR11112 does produce an M3.0 flare on Oct 16th, 2010, the implied reconnection we focus on here predates the flare by several days, and does not result in any observable flaring active of its own, such as increases in the GOES light curve, chromospheric flare ribbons, or post--flare loops. We determine that the newly emerged flux reconnects at a fairly steady average rate of 3.5e16 Mx/s over two days, while the radiated power varies between 2-8e25erg/s over the same time.
The recently-launched High-resolution Coronal imager (Hi-C) sounding rocket provided the highest resolution images of coronal loops and other small-scale structures in the 193 Angstrom passband to date. With only 5 minutes of observations, the instrument recorded a variety of dynamic coronal events -- including even a small B-class flare. We will present our results comparing these extreme-ultraviolet observations with X-ray imaging and AIA data to identify sources of hot plasma rooted in the photosphere and track their affect on the overall topology and dynamics of the active region.
We investigate the dynamics of the solar active regions by means of our data-driven time-dependent three-dimensional MHD simulation model using the HMI vector magnetic field data. The simulations start with pre-emergence phase, or very early phase of the active region so that the development of the loop structures and other signatures of the active regions will be traced. We tested several cases, mainly for AR 11158 of Feb. 2011. Either of the plasma motion or electric field, inferred from the DAVE4VM (Schuck, 2008) is given to the solar-surface boundary surface of the simulation box to which the method of projected normal characteristics (Nakagawa et al. 1987; Wu and Wang, 1987) is applied to ensure the numerical stability and consistency in physics. As our first attempt, we choose the ideal MHD equations without any additional terms except gravity. The results of the simulation show that the method can trace some signatures of the solar active regions, such as development of the magnetic-field loop and (nonlinear) twist. Not having all information at the simulation initial time, nor all physics processes on the photosphere, at transition region, and in the solar corona, agreements in plasma quantities with the other observation such as AIA image data are limited. No flare-like eruptions were obtained under a simulation setting we currently test. The temporal sequences of three-component vector data can give good constraints on the MHD simulation studies of the sub-Alfvenic region, though, we will need more observations, and probably assumptions, to fulfill the physics system. The MHD simulation can be a powerful tool to bridge the measurements and observation, helping interpretation and giving requirement.
Since 1980’s, a series of solar telescopes in China has been put into observations and got some useful data. After briefly introducing these telescopes, we will mainly describe the development of solar instruments in recent years. They are as follows: A Chinese Spectral Radio Heliograph (CSRH) is constructing at Zheng xiang bai qi, inner Mongolia. The frequency coverage is 0.4 - 15 GHz . The spatial resolution is 1.3' - 50'. The temporal resolution is better than 100 ms. CSRH has an array with 40 × 4.5 m plus 60 × 2 m parabolic antennas. The largest base line is 3 km and the field of view is 0.5 - 7 degree. The low frequency part, CSRH-I, already got the first image of the quiet Sun in Jan 2013. The high frequency part, CSRH-II, will be finished in this year. A new 1 m vacuum solar telescope (NVST) has been installed in 2010 at the observational base of YAO near the Fuxian lake, which is 60 km away from Kunming. At present it is the best seeing place in China. NVST aims at observing the sun in the range from 0.3 to 2.5 micron by high resolution imaging device and multi-wave spectrometers combined with polarization analyzer. It has obtained high resolution images at TiO, H? and other wavelengths, as well as solar spectra in optical and near infrared bands. A new telescope called ONSET (Optical and NIR Solar Eruption Tracer) has been established at the observational base of YAO in 2011. ONSET aims at studying the dynamics of flares and small activities, CME onset and its source regions, coronal structures and evolution, and white light flares. It consists of four tubes: (1) a near-infrared vacuum tube with an aperture of 27.5 cm, working at He I 10830±4.0Å with a FWHM of 0.5 Å; (2) a chromospheric vacuum tube with an aperture of 27.5 cm, working at 6562.8±2.5 Å with a FWHM of 0.25 Å; (3) a white-light vacuum tube with an aperture of 20 cm, working at the wavelength 3600Å or 4250Å with a FWHM of 15 Å; and (4) a guiding tube. ONSET can provide simultaneously images of full or partial disc (10 arcmin.) of the Sun at the three wavelengths of H? 6563Å, 10830Å, and 3600Å or 4250Å. The preliminary observations indicate that the image quality is quite good. ONSET has been put into operation since 2013.
The physical conditions such as temperature, density, and dynamical properties in the flare reconnection region, located above the bright soft X-ray loops, are basically not known although there have been measurements of non-thermal hard X-ray emission properties by RHESSI and earlier by HXT on Yohkoh. The advent of Hinode and the Solar Dynamics Observatory (SDO) spatially resolved observations, however, has changed this and it is now possible to measure in more detail some of the properties of the reconnection region. AIA imagery on SDO and the Extreme-ultraviolet Imaging Spectrometer (EIS) and X-ray Telescope (XRT) on Hinode allow values of non-thermal motions or turbulence in the reconnection region to be determined. Turbulence is predicted by theoretical models of magnetic reconnection in flares (e.g., see Liu et al. 2008, ApJ, 676, 704) and has long been inferred spectroscopically from non-thermal broadening of flare emission lines. Studies with Hinode/XRT and SDO/AIA demonstrate that two-dimensional investigations of flare velocity fields can be made, by imaging the plasma sheets above post-CME flare arcades. These measurements are made possible through the use of local correlation tracking (LCT), as shown by McKenzie (2013), ApJ, 766, 39, and reveal signatures of turbulence, including temporally and spatially varying vorticity. For some flares the AIA and XRT results can be combined with Doppler measurements of turbulence obtained with EIS. EIS data consist of raster scans that include the reconnection region for flares on the limb or near the limb. A set of spectral lines are observed that cover temperatures from 0.25 MK up to ~20 MK. A temperature in the reconnection region is calculated from the Fe XXIII/Fe XXIV line ratio and the thermal Doppler and instrumental widths are subtracted from the total line widths. The remainder is non-thermal motions or turbulence. We will present coordinated analyses of EIS and AIA observations of plasma sheets in post-CME flares, and demonstrate that the turbulent speeds found by LCT are about the same magnitude as those derived from EIS spectral line profiles obtained in the same or nearby locations.
Measurements of temperature and density near supra-arcade current sheets suggest that plasma on unreconnected field lines may experience some degree of “pre-heating” and “pre-densification” prior to their reconnection. Models of patchy reconnection allow for heating and acceleration of plasma along reconnected field lines but do not offer a mechanism for transport of energy and momentum across field lines. Here we present a model in which a reconnected flux tube retracts, deforming the surrounding layer of unreconnected field. The deformation creates constrictions that act as peristaltic pumps, driving plasma flow along affected field lines. Under certain circumstances these flows lead to shocks that can extend far out into the unreconnected field, altering the plasma properties in the affected region. These findings have direct implications for observations in the solar corona, particularly in regard to such phenomena as wakes seen behind supra-arcade downflows and high temperatures near current sheets in eruptive solar flares. This work was supported by NASA, the NSF and the DOE.
Magnetic reconnection in the corona results in contracting flare loops, releasing energy into plasma heating and shocks. These hydrodynamic shocks drive thermal conduction fronts (TCFs), which deposit energy into the chromosphere, driving upflows (evaporation) and downflows (condensation) across a range of temperatures. Observations have revealed that the transition between evaporation and condensation, the 'velocity reversal point' (VRP), occurs at a characteristic temperature and with a characteristic slope, which vary between different flares. In this study, we develop a 1-D hydrodynamical flare loop model with a simplified three-region atmosphere (chromosphere / transition region (TR) / corona), with TCFs initiated by piston shocks introduced in the corona. We investigate the effect of three different flare loop parameters (post-shock temperature, TR temperature ratio, and TR thickness) on the temperature and slope of the VRP. We find that both of the evaporation characteristics have power-law relationships to the varied flare parameters, and we report the scaling exponents for our model. Finally, we develop a method to determine the best-fit post-shock temperature and TR temperature ratio based on the observed quantities, and discuss the results for two sets of published data.
We use Hinode/XRT and SDO/AIA data to determine the thermal structure of supra arcade downflows as well as the surrounding plasma sheet. Using the multiple filters and broad temperature coverage provided by the combination of these two telescopes, we construct DEMs in the fan plasma and the supra-arcade downflows. Several models have indicated that the plasma inside the supra-arcade downflows should be significantly hotter than the surrounding plasma, but about an order of magnitude less dense. However, we find that the temperatures of the plasma within the downflows are either roughly the same as or lower than the surrounding fan plasma, with only one exception. We also compare the thermal structure of the supra-arcade plasma with calculations of the divergence of the velocity of the plasma in the sheet in order to locate evidence of adiabatic cooling and heating. The velocity fields are calculated using local correlation tracking applied to high-resolution sequences of AIA images. We find preliminary evidence that diverging velocity fields are cooler and less dense than the surrounding plasma.
Magnetic flux ropes are twisted magnetic structures. Observations of Interplanetary Coronal Mass Ejections (ICME) obtained in-situ by various spacecraft missions provide the most direct and definitive evidence of the existence of magnetic flux ropes. These structures are formed and erupted from the Sun by certain mechanisms that have been debated. One question concerns the role of magnetic reconnection in the Sun's atmosphere during the formation and eruption of flux ropes. In this study, we measure magnetic and plasma properties of 17 ICME flux ropes observed in-situ from 1998 to 2011, as well as properties of magnetic reconnection using observations of associated solar flares. We are also able to follow the motion of some CME flux ropes starting from the Sun's surface using simultaneous limb observations by STEREO. It is evident that the CME flux rope close to the Sun experiences strong acceleration when coronal reconnection rate is prominent. As reconnection helps speed up the CME, it may also transfer magnetic shear in the pre-reconnection field to magnetic field-line twist of the flux rope. We discuss this scenario by examining properties measured at the two ends of the heliosphere, from the Sun to 1 AU.
The rapid, irreversible change of the photospheric magnetic field has been recognized as an important element of the solar flare process. Recent theoretical work has shown that such a change would imply Lorentz force perturbations acting on both the outer solar atmosphere and the solar surface. In this research, we used vector magnetograms obtained with SDO/HMI to study 18 flares, which ranges from GOES-class C4 to X5 and occur in four active regions. In all the events, we found a permanent and rapid change of photospheric magnetic field closely associated with the flare occurrence. The change is predominantly in the form of an enhancement of the horizontal magnetic field, which is located around the magnetic polarity inversion line usually between flare ribbons. The change of field integrated over the area and the derived change of Lorentz force both show a strong correlation with flare magnitude. For seven events with associated coronal mass ejections (CMEs), we used an estimate of the impulse provided by the upward Lorentz force, plus the observed CME velocity, to estimate the CME mass. Furthermore, we calculated the flow field vorticity of selected sunspots away from flare kernels in the AR 11158, using the Differential Affine Velocity Estimator. We found that some spots exhibit a sharp acceleration of rotation co-temporal with the rapid rising of the soft X-ray flux, and that such rotational disturbance may be driven by the Lorentz-force change in the horizontal direction.
The solar atmosphere may be heated by Alfven waves that propagate up from the convection zone and dissipate their energy in the chromosphere and corona. To further test this theory, we consider wave heating in an active region observed on 2012 March 7. A potential field model of the region is constructed, and 22 field lines representing observed coronal loops are traced through the model. Using a three-dimensional (3D) reduced magneto-hydrodynamics (MHD) code, we simulate the dynamics of Alfven waves in and near the observed loops. The results for different loops are combined into a single formula describing the average heating rate $Q$ as function of position within the observed active region. We suggest this expression may be approximately valid also for other active regions, and therefore may be used to construct 3D, time-dependent models of the coronal plasma. Such models are needed to understand the role of thermal non-equilibrium in the structuring and dynamics of the Sun's corona.
We present a study of the frequency and duration of brightenings in the core of solar active regions as observed in the Fe XVIII line component of AIA/SDO 94 A filter images. The Fe XVIII emission was isolated by removing the 'warm' emission contribution using as proxy the emission from the AIA 193 and 171 channels. We examined the evolution of loop in cores of several active regions that span a wide range of total magnetic field strengths and at various stages of evolution. Using a newly developed event detector algorithm we find that the typical frequency of occurrence of detectable brightness enhancements is in the order of 20 minutes. Using EBTEL, a 0D hydrodynamical model, we show that a single loop heated a that frequency would be experiencing effectively steady heating. Then we evaluate different heating scenarios with multiple loops along the line-of-sight. Finally, we report on our preliminary efforts to reproduce those characteristic timescales on full active region models where field lines from a non-linear force free extrapolation are populated with EBTEL solutions.
Observations of comets occupy a rich history within Solar and Heliospheric science. Cometary plasma tails probe the solar wind in the inner solar system (~0.5-3 AU) and their observations led to its discovery more than half a century ago. Fast forwarding to today, recent observations of sun-grazing comets within the solar corona have opened up a whole new avenue to study the Sun with these striking celestial bodies. Here we present our recent study of the perihelion passage of comet C/2011 W3 (Lovejoy), which came within 140Mm of the solar surface. Imaged from multiple perspectives by SDO/AIA and the STEREO/EUVI, extreme ultraviolet (EUV) observations of Lovejoy's tail showed substantial changes in direction, intensity, magnitude, and persistence. To understand this unique signature, we combine a state-of-the-art magnetohydrodynamic (MHD) model of the solar corona and a prescription for the motion of emitting cometary tail ions in an embedded plasma. We show how the observed tail motions reveal the inhomogeneous magnetic field of the solar corona, and demonstrate how they constrain field and plasma properties in a region where the coronal plasma is normally not easily observed in EUV. We will also discuss our results in context of the upcoming perihelion passage of comet C/2012 S1 (ISON), expected by many to be a spectacular probe of the near-sun environment. Work supported by NASA and NSF.
MHD simulations of the solar corona rely on maps of the solar magnetic field (typically measured at the photosphere) for input as boundary conditions. These 'synoptic' maps (available from a number of ground-based and space-based solar observatories), which are perhaps better described as 'diachronic,' are built up over a solar rotation. A well-known problem with this approach is that the maps contain data that is as much as 27 days old. The Sun's magnetic flux is always evolving, and these changes in the flux affect coronal and heliospheric structure. Flux evolution models can in principle provide a more accurate specification, by estimating the likely state of the photospheric magnetic field on unobserved portions of the Sun. The Air Force Data Assimilative Photospheric flux Transport (ADAPT) model (Arge et al. 2010), which incorporates data assimilation techniques into the Worden and Harvey (2000) flux evolution model, is especially well-suited for this purpose. In this presentation we describe the use of such 'synchronic' maps with coronal models. We compare results using synchronic maps versus the traditional synoptic maps. Research supported by AFOSR, NASA, and NSF.
The Advanced Technology Solar Telescope (ATST) will provide observing capabilities in the visible through infrared wavelengths with unprecedented resolution and sensitivity. Designed to study solar magnetism that controls the solar wind, flares, CMEs and variability in the Sun's output, the ATST will be capable of detecting and spatially resolving the fundamental astrophysical processes at their intrinsic scales throughout the solar atmosphere. The 4-m class facility is currently under construction in Maui, HI on the Haleakala Observatories site with a scheduled completion of July 2019. Since the start of site construction in December of 2012, significant progress has been made toward the development of the observatory buildings (excavation, foundations, working towards the steel erection). In addition, off-site, the major subsystems of the telescope have been contracted, designs are complete and fabrication is underway. We review the science drivers, design details, technical challenges, and provide a construction status update on the subsystems and their integration.
The ATST will have a full complement of first generation instrumentation to cover observations of the solar atmosphere from the photosphere to the corona: the Visible Broadband Imager (VBI) will be an interference filter imager providing the highest spatial and temporal resolution image sequences for ATST, spanning from the deep photosphere through the chromosphere, and perhaps providing coronal imaging as well; the Visible Spectropolarimeter (ViSP) will be an advanced slit spectropolarimeter enabling simultaneous multi-line spectropolarimetry from 380 to 900 nm; the Visible Tunable Filter (VTF) will be a dual tunable Fabry-Perot system enabling rapid cadence spectral imaging and spectropolarimetry in the 520 to 870 nm range; the Diffraction-limited Near-IR Spectropolarimeter (DL-NiRSP) will be an advanced fiber-optic image plane spectropolarimeter offering simultaneous imaging and full-profile spectropolarimetry from 900 to 2500 nm; and the Cryogenic Near-IR Spectropolarimeter (Cryo-NiRSP) will provide coronal slit spectropolarimetry from 1--5 microns. We will review the science capabilities of these first generation instruments as well as their synergistic use in multi-instrument observing programs to achieve novel science investigations.
The ATST will generate a large volume of data simultaneously from multiple instruments. These data will have value both to the investigators who design specific observational programs, and as an archival dataset. The challenges in processing ground-based, high-resolution observations are many, and the need to produce robust processing pipelines for heterogeneous datasets compound the difficulties. I will discuss some approaches for efficiently managing these data and generating useful data products. I will also describe opportunities for the community to contribute to the planning and preparations for the ATST data stream.
The Advanced Technology Solar Telescope (ATST) will serve the US international community for at least two decades. As such it is required to be a facility that allows for the flexibility to address a large number of diverse scientific questions in the most efficient way not only during the first years of operations but also in the long term. In order to cope with these challenging requirements the ATST needs to adopt new strategies and procedures touching all aspects of operations. We would like to present some of the conceptual strategies envisaged for the ATST that address the above challenges but will especially concentrate on how scientific observing requests (proposals) are realized (or executed) at the facility on a daily basis.
The ATST will provide unprecedented measurements of small-scale fields and flows in the solar photosphere and chromosphere, and what we learn at those scales will have implications for models of global solar behavior. We will discuss these connections in the context of two important problems: the operation of the global solar dynamo and the variability of the solar spectral irradiance. For both of these, measuring the statistical properties of small-scale magnetic flux elements and their dynamics is critical. ATST will allow exploration of the small-scale magnetohydrodynamics that underlies the turbulent diffusion processes central to dynamo behavior. ATST will also allow characterization of the magnetic substructure that underlies variation in spectral irradiance. In both cases what we learn about the small scales will have global impacts that can be studied only by including their contributions in global models statistically. Arriving at such statistical descriptions poses a compelling challenge, which we have only begun to address.
The 1.6 m, off-axis, clear aperture New Solar Telescope (NST) has been in regular operation in Big Bear Solar Observatory since 2009. The NST is the first facility class solar telescope built in the U.S. in a generation, which already offers a significant improvement in ground-based high angular resolution capabilities. This presentation reports the up-to-date progress on the NST and its 2nd generation instruments including the AO system (AO-308), the Near-InfraRed Imaging Spectro-polarimeter (NIRIS), the Visible Imaging Spectrometer (VIS), and the Cryogenic Infrared Spectrograph (CYRA).
The ATST project recently began construction on Haleakala, Maui. When combined with the dark daytime sky conditions at the site, the low scattered light primary mirror should allow ATST to make coronal observations at infrared wavelengths. These data will address several coronal science topics included in the ATST Science Requirements Document. I will review the early work done by the ATST Coronal Science Working Group, including these coronal science use cases and the expected background-limited measurement errors. In summary, the Working Group found that given good sky conditions the 1-sigma measurement errors in a 40 millionths brightness corona were 0.04 millionths for intensity, 9 x 10^4 cm^-3 for electron density, 37 m s^-1 for velocity and 33 Gauss for the magnetic field as measured in a 1 arcsecond square pixel with a 1 second exposure using the infrared coronal emission lines at 1075nm. These measurement errors increase as the telescope or sky backgrounds increase. Finally I will update these error estimates from the latest telescope models, and discuss recent coronal science topics.
A flare's radiation appears mostly in the near-UV, optical and near infrared, emitted by the dense chromosphere where most of the flare energy is ultimately dissipated. Catching flares with a small field-of-view imager or a slit-rastering spectrometer is challenging, but observations with the ATST will lead to tremendous advances in our knowledge of a flare's magnetic environment and its variations, the structure and evolution of the flare chromosphere, and the temporal and spatial scales of energy transport and dissipation. This talk will review in brief our understanding of flares in the ATST's wavelength range, and describe some observational goals for flare science with the ATST first light instruments.
So called 'realistic' magnetoconvection simulations of the solar photosphere include all relevant physical ingredients in terms of equation of state and 3 dimensional radiative transfer in addition to MHD. In that sense they do not have any explicit free parameters, however, implicit degrees of freedom are present since simulations are limited to a finite volume, have a finite resolution and can be only run for a finite time. This results in dependencies on boundary conditions, on the numerical treatment of unresolved scales and on the chosen initial state. A successful numerical model of the solar photosphere and underlying convection zone is only possible if these implicit degrees of freedom are sufficiently constrained through observations. In this talk I will discuss two examples of recent high resolution simulations: the quiet sun photosphere and sunspot fine structure. Numerical simulations of quiet sun magnetism can explain most of the observed unsigned magnetic flux density as a consequence of a small scale dynamo process. These simulations make however unrealistic assumptions about the small scale dissipation and depend to some degree on the assumed bottom boundary condition several Mm beneath the photosphere. Observations are needed to verify the validity of this modeling approach. Sunspot simulations have successfully linked convective energy transport in the penumbra with penumbral fine structure. Current observations provide several indirect hints on convective flows, but higher resolution is needed to settle this aspect. So far sunspot simulations cannot explain from first principles the existence and extent of a penumbra, since this aspect depends strongly on the magnetic top boundary condition. Observations of the detailed magnetic field structure in the upper photosphere, chromosphere and lower corona above sunspots are needed to guide modeling. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
The Chinese Giant Solar Telescope (CGST) will be a diffraction limited solar telescope optimized for the near-infrared (NIR) spectral region (0.8 – 2.5 microns). Its diffraction limit will be reached by the incorporation of Multi-Conjugate Adaptive Optics (MCAO) enhanced by image restoration techniques to achieve uniform (u.v) plane coverage over the angular spatial frequency region allowed by its 8-meter aperture. Thus it will complement the imaging capabilities of 4-meter telescopes being planned elsewhere which are optimized for the visible (VIS) spectral region (300 – 1000 nm) In the NIR spectral regions the CGST will have access to unique spectral features which will improve the diagnostics of the solar atmosphere. These include the CaII lines near 860 nm , the HeI lines near 1083 nm, the 1074 nm FeXIII coronal lines, the large Zeeman-split FeI line at 1548 nm, and (v) the H- continuum absorption minimum at 1.6 micron. Especially in sunspot umbrae the simultaneous observation of continua and lines across the NIR spectral range will cover a substantial depth range in the solar atmosphere. Of course the mid- and far- infrared regions are also available for unequalled high-angular resolution solar observations, for example, in the Hydrogen Bracket lines, CO molecular bands, and the MgI emission line at 12.3 microns. The CGST is a so-called ring telescope in which the light is captured by a 1 meter wide segmented ring or by a ring of 7 smaller off-axis aperture telescopes. The open central area of the telescope is large. The advantages of such a ring configuration is that (a) it covers all the spatial frequencies out to those corresponding to its outer diameter, (b) its circular symmetry makes it polarization neutral, (c) its large central hole helps thermal control, and (d) it provides ample space for the MCAO system and instrumentation in the Gregorian focus. Even though optimized for the NIR, we expect to use the CGST also at visible wavelengths in the so-called “Partial Adaptive Optics” (PAO) mode (Applied Optics 31,424,1992) to obtain angular resolution twice that of a 4-meter telescope if their observations indicate that higher resolution is desirable. The CGST is a Chinese solar community project.
Up to tens of percent of the energy released in solar flares goes into accelerating electrons above ~10 keV and ions above ~1 MeV, and the impulsive heating of the ambient solar atmosphere by these particles is partially or wholly responsible for the production of hot flare plasmas (up to ~50 MK). Although flares can accelerate electrons to relativistic energies, in even large flares the typical falling power-law energy spectrum means that the plasma is primarily heated by the much larger number of low-energy electrons. However, there have been flares observed where the electron energy spectra have high low-energy cutoffs (well above ~100 keV), which significantly changes the electron energies responsible for heating and modifies the usual conception of energy transport in a flare. A systematic study of a range of relativistic-electron-dominated flares can improve our understanding of the relevant acceleration processes and how they may differ from those in 'typical' flares. We search the Fermi/GBM data set for such flares based on the electron-associated X-ray/gamma-ray bremsstrahlung emission, making use of an improved background-subtraction approach to improve the ability to detect weaker flares. We present the fitted parameters for the relativistic-electron spectrum and their evolution over time, and compare against RHESSI observations and other instruments when available. We also discuss these events in the context of previously observed correlations between relativistic-electron acceleration and ion acceleration in flares.
The Fermi Large Area Telescope (LAT) is the most sensitive instrument ever deployed in space for observing high-energy gamma-ray emission. It has now observed 19 solar flares above 100 MeV in four years of mission. A few flares are detected only in their impulsive phase, but most of them show sustained emission up to GeV energies lasting from a few to 20 hours. General characteristics of the gamma-ray emission during these flares will be presented. Interestingly all flares are associated with fairly fast coronal mass ejections and solar energetic particles, which could shed some light on the acceleration processes of particles at the highest energies. We will also present the detailed analyses, including temporal and spectral features, of the longest and brightest flares detected by the Fermi LAT so far, the 2012 March 7 X-class flares.
We present Nancay Radioheliograph observations of a moving type IV solar radio burst which occurred in association with a CME on the 14th of August 2010. The event was well observed at extreme ultraviolet wavelengths by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, the SWAP instrument onboard Proba2 and by the LASCO white light coronograph. The burst emission was found to be cospatial with the core of the CME. Using radio imaging spectroscopy we are able to characterize the underlying electron distribution and plasma parameters within the source. Fitted spectra reveal a clear power law component consistent with optically thin synchrotron emission from accelerated electrons trapped in the erupting flux rope. As is often observed in type IV bursts, polarization measurements show the source to be moderately polarized during the peak of the burst, before steadily increasing to around 70% as the brightness temperature of the burst decays.
The limb flare of 2012 July 19 around 5UT provides the best example of a non-thermal above-the-loop-top hard X-ray source with simultaneous observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamic Observatory (SDO). Combining the two sets of observations we present the first direct measurement of the thermal proton density and non-thermal electron density within the above-the-loop-top source of a solar flare. We find that both densities are of the same order of magnitude of a few times 1e9 cm-3, about 30 times lower than the density in the underlying thermal flare loops. The equal densities indicate that the entire electron distribution within the above-the-loop-top source is energized. While the derived densities depend on the unknown source depth and filling factor, the ratio of these two densities does not. Within the uncertainties, the ratio is one for a low energy cut-off of the non-thermal electron spectrum between 10 and 16 keV, consistent with equal densities. RHESSI observations only constrain the cut-off energy to below 20 keV, leaving the spectral shape of the electrons within the above-the-loop-top source below 20 keV unknown. Nevertheless, these robust results strongly corroborate earlier findings that the above-the-loop-top source is the acceleration region where a bulk energization process acts on all electrons.
Hard X-ray (HXR) spikes refer to fine time structures on timescales of seconds to milliseconds in high-energy HXR emission profiles during solar flare eruptions. We present a preliminary statistical investigation of temporal and spectral properties of HXR spikes. Using a three-sigma spike selection rule, we detected 184 spikes in 94 out of 322 flares with significant counts at given photon energies, which were detected from demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are also detected at photon energies higher than 100 keV. The statistical properties of the spikes are as follows. (1) HXR spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. Ninety percent of the spikes occur during the rise phase of the flares, and about 70% occur around the peak times of the flares. (2) The time durations of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on photon energies. The spikes exhibit symmetric time profiles with no significant difference between rise and decay times.(3) Among the most energetic spikes, nearly all of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions.
One of the last unexplored wavelength frontiers for solar flares is in the range of submillimeter to infrared wavelengths. We report the detection of an intense impulsive burst at 30 THz using a new imaging system. The 30 THz emission exhibited remarkable time coincidence with peaks observed at microwave, mm/submm, visible, EUV and hard X-ray wavelengths. The 30 THz burst location matches a weak white-light feature, an intense EUV knot, and a hard X-ray source. The two spatial structures at EUV are not time coincident, and appear to correspond to two successive peaks at 30 THz and submm, the second one without time correspondence with the impulsive component. The coincidence with a white-light feature is consistent with heating below the temperature minimum in the atmosphere. However, there are problems in attributing the heating to accelerated electrons. The peak 30 THz flux is several times larger than the usual microwave peak near 9 GHz, attributed to non-thermal electrons in the corona. The 30 THz emission could be consistent with an optically thick spectrum increasing from low to high frequencies. It might be part of the same spectral component found at sub-THz frequencies whose nature remains mysterious. Further observations at these wavelengths will provide a new window for flare studies.
The polar fields on the Sun are directly related to solar cycle variability. Recently there has been interest in studying an important characteristic of the polar fields: the timing of the polar field reversals. However this characteristic has been poorly defined, mostly due to the limitations of early observations. In the past, the reversals have been calculated by averaging the flux above some latitude (i.e. 55° or 75°). Alternatively, the reversal could be defined by the time in which the previous polarity is completely canceled and replaced by the new polarity at 90°, precisely at the pole. We will use a surface flux transport model to illustrate the differences in the timing of the polar field reversal based on each of these definitions and propose standardization in the definition of the polar field reversal. The ability to predict the timing of the polar field reversal using a surface flux transport model will also be discussed.
In the simplest of forms, modern dynamo theory describes the solar cycle as a process that takes the solar magnetic field (back and forth) from a configuration that is predominantly poloidal (contained inside the meridional plane), to one predominantly toroidal (wrapped around the axis of rotation). However, there is still uncertainty and controversy in the detailed understanding of this process. A major contributor to this uncertainty is the lack of direct long-term databases covering different components of the solar magnetic field (an issue mainly affecting the poloidal component of the solar magnetic field). In this talk we will review the different observations that can be used as proxies for the solar magnetic field (in absence of direct magnetic observations). I will present a recently standardized database that can be used as a proxy for the evolution of the polar magnetic field. And to conclude, I will show the insights that can be gained (by taking advantage of this database) in the context of the transition between the toroidal and poloidal phases of the cycle, solar cycle memory as determined by the different mechanisms of flux transport, and the practical goal of solar cycle prediction.
It was long assumed that the meridional circulation on the Sun is represented by a single cell occupying the whole convection zone, with poleward flow at the top and with the return equator-ward flow at the bottom. However, recent helioseismology observations and numerical simulations provided clear evidence that the meridional circulation has a double-cell structure with a return equator-ward flow in the middle of the convection zone. This discovery requires to re-examine the solar dynamo models. We discuss the properties of a new mean-field solar dynamo that is coupled with the double-cell meridional circulation pattern. It is found that such dynamo model (which also includes the subsurface rotational shear layer, turbulent pumping and other turbulence effects) can robustly reproduce the basic properties of the solar magnetic activity within the wide range of the dynamo parameters and amplitudes of the circulation speed. The properties of the simulated sunspot activity migration are discussed and compared with observations. It is found that the best agreement with observation is achieved when the surface speed of circulation is about 12 m/s. Interesting that for this amplitude of the circulation speed the simulated sunspot activity show the pretty good synchronization with the polar magnetic field activity. Such synchronization was indeed observed during the past Cycles 21 and 22. We compare our findings with these observations.
Kinematic dynamo models of the solar cycle that rely on magnetic flux transport processes have traditionally relied on a deep meridional flow pervading the full solar convection zone. The current understanding of the solar cycle, based not only on such dynamo models but also on flux tube dynamics simulations, is that strong toroidal flux tubes are stored and amplified in a stable layer beneath the base of the convection zone, from where they buoyantly rise to produce sunspots. However, some recent interpretations of solar meridional flow observations are along the lines that the meridional flow is shallow and remains confined to the top 10% of the Sun. Here, we explore whether flux transport dynamos could function with such a shallow meridional flow and discuss the consequences that this scenario would have on our traditional understanding of magnetic field dynamics in the solar interior.
Fluctuations in solar magnetic output, including episodes of grand minima such as the Maunder minimum are studied using a mathematical model based on time delay differential equations. Time delays physically capture the effect of the finite time required for magnetic flux transport between two spatially segregated source regions for toroidal and poloidal field creation. We show that cycle to cycle variations and Maunder-like grand minima are naturally produced by introduction of stochastic fluctuation in the poloidal field generation mechanism. Our analysis shows that the Babcock-Leighton mechanism, alone, cannot restart the sunspot cycle once it settles into a grand minimum. An additional poloidal field source capable of working on weak toroidal magnetic fields - such as that driven by helical turbulence (mean-field alpha-effect) - is necessary to recover the solar cycle. Our result demonstrates the importance of a small-scale alpha-effect in the context of the large-scale solar cycle dynamics and shows, how self consistent entry and exit from grand minima like episodes are possible.
An array of methods have been developed over the past few decades aimed at inferring the surface motion in the solar photosphere. These methods are generally based on tracking the apparent motion of features seen in the data which are, for the most part, manifestations of the thermal or magnetic structuring generated by solar magnetoconvection. Patterns formed by nonlinear magnetoconvection are known change dramatically depending on the configuration and strength of the magnetic field. These changes should be taken into account in assessing the performance of any flow-tracking method. Here we assess one method using high-fidelity numerical models of the magnetoconvection in the presence of a large-scale region of emerging flux. We compare the flow structure derived from the opflow3d method against the surface velocities contained within the simulation and investigate systematic errors introduced by local variations in field strength and inclination.
Motivated by available observations of two different flares in Ly? and H?, we model the conditions of the solar atmosphere using a radiation hydrodynamics code (RADYN, Carlsson & Stein, 1992) and analyze the energy transport carried by a beam of non-thermal electrons injected at the top of a 1D coronal loop. The numerical Ly? and H? intensities match with the observations. The electron energy distribution is assumed to follow a power law of the form (E/Ec )-? for energies greater than a cutoff value of Ec. Abbett & Hawley (1999) and Allred et al. (2005) assumed that the non-thermal electrons flux injected at the top of a flaring loop, the cut-off energy and the power law index are constant over time. An improvement was achieved by Allred & Hawley (2006), who modified the RADYN code in such a way that the input parameters were time dependent. Their inputs were based on observations of a flare obtained with RHESSI. By combining RADYN with the “flare” code from Stanford University which models the acceleration and transport of particles and radiation of solar flares in non-LTE regime, we can calculate the non-thermal electrons flux, the cut-off energy and the power law index at every simulated time step. The atmospheric parameters calculated by RADYN could in turn be used as updated inputs for 'flare', providing several advantages over the results from Liu et al. (2009), who combined the particle acceleration code with a 1-D hydrodynamic code, improving the atmospheric conditions.
The most common practice of the determination of the characteristics of the radiation and acceleration mechanisms in solar flares are through forward fitting procedures where observed spectra are fitted to those obtained from assumed parametric model. The uniqueness of the result is always questionable. Here we describe a new 'Inversion' method for a non-parametric determination of energy dependences of critical characteristics of the acceleration mechanism in the framework of the Fokker-Planck kinetic equation. The assumptions leading to this model and the applicability of the inversion method to both stochastic acceleration by turbulence and/or shock acceleration will be described. We then will present results from application of the method to several solar flares observed by RHESSI with distinct bremsstrahlung hard X-ray emission from the looptop and thick-target footpoint regions of the flaring loop. We present results based on either the so-called regularized inversion from visibility to electron spectra or the traditional hard X-ray imaging spectroscopy. The result show some puzzling aspects for both shock or stochastic acceleration models implications of which will be discussed. We will also briefly discuss potential application of the method to particle acceleration in supernova remnants.
Hard X-ray (HXR) sources located in the corona may be regarded as evidence for the collisionally thick target in the corona. We investigated whether this idea can be independently verified by microwave radiations that have been known as the best companion to HXRs. Although the sensitivity of microwave radiations to magnetic field has been of main emphasis, there are two other spectral features sensitive to thermal density, the Razin suppression and free–free emission. These density sensitive features may not be significant at typical coronal densities, but become prominent at very high densities that are considered necessary for the collisionally thick target model. In addition, we stress that there is an absolute cut-off of microwave radiation at the plasma frequency which is a sole function of thermal density. These three spectral features make microwave spectral observations an essential tool for testing the thick target models for coronal HXR sources. The test has been made for the 2002 September 9 flare that was observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the Owens Valley Solar Array.
We present an unprecedented high-resolution halpha imaging spectroscopic observation of a C4.1 flare taken with IBIS on 2011 October 22. The flare consists of a main circular ribbon that occurred in a parasitic magnetic configuration and a remote ribbon that was observed by the IBIS. Such a circular-ribbon flare with a remote brightening is predicted in 3D fan-spine reconnection but so far has been rarely reported. During the flare impulsive phase, we define 'core' and 'halo' structures in the observed ribbon. Examining the halpha emission spectra averaged in the flare core and halo areas, we find that only those from the flare cores show typical nonthermal electron beam heating characteristics. These characteristics include: broad and centrally reversed emission spectra, excess emission in the red wing with regard to the blue wing (i.e., red asymmetry), and redshifted bisectors of the emission spectra. We also observe rather quick timescales for the heating (30 s) and cooling (14--33 s) in the flare core locations. Therefore, we suggest that the flare cores revealed by IBIS track the sites of electron beam precipitation with exceptional spatial and temporal resolution. The flare cores show two-stage motion (a parallel motion along the ribbon followed by an expansion motion perpendicular to the ribbon) during the two impulsive phases of the flare. Some cores jump quickly (30 km/s) between discrete magnetic elements implying reconnection involving different flux tubes. We observe a very high temporal correlation (>0.9) between the integrated halpha and HXR emission during the flare impulsive phase. A short time delay (4.6 s) is also found in the halpha emission spikes relative to HXR bursts. The ionization timescale of the cool chromosphere and the extra time taken for the electrons to travel to the remote ribbon site may contribute to this delay.
Solar flares involve catastrophic release of magnetic energy previously stored in the Sun's corona. This dissertation focuses on studies of radio and hard X-ray emissions as diagnostics of energy release in flares. A major part of the dissertation is exploiting spatially resolved dynamic spectroscopy to study coherent radio bursts. The Frequency-Agile Solar Radiotelescope Subsystem Testbed, a three-element radio interferometer, provides the first opportunity of doing such studies on zebra-pattern bursts. The observations allow us to identify the relevant emission mechanism, enabling diagnostics of the plasma parameters in the source. With the help of coronal magnetic field extrapolations, the source is placed into a three-dimensional magnetic field configuration and its relation to the energy release is clarified. The next part of the dissertation discusses the 'solar mode' commissioning of the upgraded Karl G. Jansky Very Large Array (VLA). As a general purpose telescope, special provisions should be made for the VLA to enable solar observations. Based on the test results on the VLA's hardware, solar observing and calibration strategies are developed. Now the VLA is capable of observing the Sun with simultaneous imaging and dynamic spectroscopy over a large bandwidth at high spatial, spectral, and temporal resolutions. The upgraded VLA is used to observe decimetric type III radio bursts, which are the radio signature of propagating fast electron beams produced in flares. The new observing technique allows detailed trajectories of these electron beams to be derived. Combined with multi-wavelength observations, the properties of the energy release site, electron beams, and the surrounding coronal medium are deduced. The dissertation also presents a study on coronal hard X-ray/gamma-ray sources. Rather extreme conditions are needed to account for some observed coronal hard X-ray/gamma-ray sources using the usually-assumed non-thermal bremsstrahlung emission. This study investigates whether inverse Compton scattering could be an alternative emission mechanism for these sources, which would open a new window in diagnosing the flare energy release.
We present an investigation of the large-scale flows that influence magnetic fields at the solar surface. The aim of this work is to accurately characterise the supergranular diffusion coefficient, D, that governs the dispersal rate of magnetic features in the photosphere. There is a disconnect between the measured rate of magnetic field dispersal (~50 - 300 km2/s) and the value of D used in global simulations of solar magnetic field evolution (~500 - 600 km2/s). We track the poleward motion of magnetic features in a latitude-time map and compare the poleward progression to a data-driven simulation that includes differential rotation, the meridional flow, and supergranular diffusion. We find that over a time scale of months, setting D = 100 km2/s matches observations, but over a time scale of years, setting D = 500 km2/s is a better match. This supports the idea that observational time scale causes the disconnect in D values, which leads us to the conclusion that the present magnetic surface flux transport model is not adequate to explain the observed evolution of the solar surface magnetic field.
Helioseismology can be an important tool for understanding the formation of active regions. As a first step towards this goal, we have carried out a search for statistically significant helioseismic precursors of active region emergence. We used an automatic method to determine the time of emergence based on the NOAA/NGDC active region catalog and MDI/SOHO 96 minute magnetograms. Using GONG data, we applied helioseismic holography to 107 pre-emergence active regions and a control sample of 107 regions where no active region was present. We found some significant and surprising differences between our samples in both quantities determined from helioseismology and from surface magnetic fields. However, we do not see a clear signature of emergence when considering individual active regions. The results of this investigation may shed some light on the mechanism responsible for flux emergence, and certainly illustrate the care which must be taken in conducting such an investigation. This work was supported by NASA contract NNH07CD25C.
In the context of local helioseismology, we use the first-order Born approximation to compute 3D sensitivity kernels due to perturbations in sound-speed, density or pressure, with respect to a horizontally-invariant background solar model.
Among many techniques of local helioseismology, the ring-diagram method has been quite popular because of its ability to quickly analyze vast amounts of high-resolution data. In ring analysis, the speed and direction of horizontal flows beneath the solar surface are assessed by inverting fitted surface velocities for a given set of modes. Here we discuss the validationof the inversion technique through the use of supergranulation scale hydrodynamic numerical simulations.