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Effect of transport coefficients on excitation of flare-induced standing slow-mode waves in coronal loops  

Tongjiang Wang   Submitted: 2018-05-10 14:02

Standing slow-mode waves have been recently observed in flaring loops by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). By means of the coronal seismology technique transport coefficients in hot (∼10 MK) plasma were determined by Wang et al.(2015, Paper I), revealing that thermal conductivity is nearly suppressed and compressive viscosity is enhanced by more than an order of magnitude. In this study we use 1D nonlinear MHD simulations to validate the predicted results from the linear theory and investigate the standing slow-mode wave excitation mechanism. We first explore the wave trigger based on the magnetic field extrapolation and flare emission features. Using a flow pulse driven at one footpoint we simulate the wave excitation in two types of loop models: model 1 with the classical transport coefficients and model 2 with the seismology-determined transport coefficients. We find that model 2 can form the standing wave pattern (within about one period) from initial propagating disturbances much faster than model 1, in better agreement with the observations. Simulations of the harmonic waves and the Fourier decomposition analysis show that the scaling law between damping time (τ) and wave period (P) follows τ∝P2 in model 2, while τ∝P in model 1. This indicates that the largely enhanced viscosity efficiently increases the dissipation of higher harmonic components, favoring the quick formation of the fundamental standing mode. Our study suggests that observational constraints on the transport coefficients are important in understanding both, the wave excitation and damping mechanisms.

Authors: Tongjiang Wang, Leon Ofman, Xudong Sun, Sami K Solanki, Joseph M Davila
Projects: SDO-AIA,SDO-HMI

Publication Status: Accepted by ApJ
Last Modified: 2018-05-11 16:11
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Variation of Coronal Activity from the Minimum to Maximum of Solar Cycle 24 using Three Dimensional Coronal Electron Density Reconstructions from STEREO/COR1  

Tongjiang Wang   Submitted: 2017-06-19 14:15

Three dimensional electron density distributions in the solar corona are reconstructed for 100 Carrington Rotations (CR 2054-2153) during 2007/03-2014/08 using the spherically symmetric method from polarized white-light observations with the STEREO/COR1. These three-dimensional electron density distributions are validated by comparison with similar density models derived using other methods such as tomography and a MHD model as well as using data from SOHO/LASCO-C2. Uncertainties in the estimated total mass of the global corona are analyzed based on differences between the density distributions for COR1-A and -B. Long-term variations of coronal activity in terms of the global and hemispheric average electron densities (equivalent to the total coronal mass) reveal a hemispheric asymmetry during the rising phase of Solar Cycle 24, with the northern hemisphere leading the southern hemisphere by a phase shift of 7-9 months. Using 14-CR (~13-month) running averages, the amplitudes of the variation in average electron density between Cycle 24 maximum and Cycle 23/24 minimum (called the modulation factors) are found to be in the range of 1.6-4.3. These modulation factors are latitudinally dependent, being largest in polar regions and smallest in the equatorial region. These modulation factors also show a hemispheric asymmetry, being somewhat larger in the southern hemisphere. The wavelet analysis shows that the short-term quasi-periodic oscillations during the rising and maximum phases of Cycle 24 have a dominant period of 7-8 months. In addition, it is found that the radial distribution of mean electron density for streamers at Cycle 24 maximum is only slightly larger (by ~30%) than at cycle minimum.

Authors: Tongjiang Wang, Nelson L. Reginald, Joseph M. Davila, O. Chris St. Cyr, William T. Thompson
Projects: STEREO

Publication Status: Accepted for publication in Solar Physics, in June 2017
Last Modified: 2017-06-20 16:06
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Waves in Solar Coronal Loops  

Tongjiang Wang   Submitted: 2016-02-25 17:54

The corona is visible in the optical band only during a total solar eclipse or with a coronagraph. Coronal loops are believed to be plasma-filled closed magnetic flux anchored in the photosphere. Based on the temperature regime, they are generally classified into cool, warm, and hot loops. The magnetized coronal structures support propagation of various types of magnetohydrodynamics (MHD) waves. This chapter reviews the recent progress made in studies based on observations of four types of wave phenomena mainly occurring in coronal loops of active regions, including: flare-excited slow-mode waves; impulsively excited kink-mode waves; propagating slow magnetoacoustic waves; and ubiquitous propagating kink (Alfvénic) waves. This review not only comprehensively discusses these waves and coronal seismology but also topics that are newly emerging or hotly debated in order to provide the reader with useful guidance on further studies.

Authors: Wang, Tongjiang
Projects: None

Publication Status: Wiley Book, Published Online: 12 FEB 2016
Last Modified: 2016-02-29 10:55
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Evidence of thermal conduction suppression in hot coronal loops: Supplementary results  

Tongjiang Wang   Submitted: 2015-10-13 11:38

Slow magnetoacoustic waves were first detected in hot (>6 MK) flare loops by the SOHO/SUMER spectrometer as Doppler shift oscillations in Fe XIX and Fe XXI lines. Recently, such longitudinal waves have been found by SDO/AIA in the 94 and 131 Å channels. Wang et al. (2015) reported the first AIA event revealing signaturesin agreement with a fundamental standing slow-mode wave, and found quantitative evidence for thermal conduction suppression from the temperature and density perturbations in the hot loop plasma of > 9 MK. The present study extends the work of Wang et al. (2015) by using an alternative approach. We determine the polytropic index directly based on the polytropic assumption instead of invoking the linear approximation. The same results are obtained as in the linear approximation, indicating that the nonlinearity effect is negligible. We find that the flare loop cools slower (by a factor of 2-4) than expected from the classical Spitzer conductive cooling, approximately consistent with the result of conduction suppression obtained from the wave analysis. The modified Spitzer cooling timescales based on thenonlocal conduction approximation are consistent with the observed, suggesting that nonlocal conduction may account for the observed conduction suppression in this event. In addition, the conduction suppression mechanism predicts that larger flares may tend tobe hotter than expected by the EM-T relation derived by Shibata & Yokoyama (2002)

Authors: Tongjiang Wang, Leon Ofman, Xudong Sun, Elena Provornikova, Joseph M. Davila
Projects: SDO-AIA

Publication Status: submitted to Proceedings of IAU29 GA Symposium 320 (2015)
Last Modified: 2015-10-15 07:27
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Evidence of thermal conduction suppression in a solar flaring loop by coronal seismology of slow-mode waves  

Tongjiang Wang   Submitted: 2015-09-08 13:23

Analysis of a longitudinal wave event observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. A time sequence of 131 Å images reveals that a C-class flare occurred at one footpoint of a large loop and triggered an intensity disturbance (enhancement) propagating along it. The spatial features and temporal evolution suggest that a fundamental standing slow-mode wave could be set up quickly after meeting of two initial disturbances from the opposite footpoints. The oscillations have a period of ~12 min and a decay time of ~9 min. The measured phase speed of 500±50 km s-1 matches the sound speed in the heated loop of ~10 MK, confirming that the observed waves are of slow mode. We derive the time-dependent temperature and electron density wave signals from six AIA extreme-ultraviolet (EUV) channels, and find that they are nearly in phase.The measured polytropic index from the temperature and density perturbations is 1.64±0.08 close to the adiabatic index of 5/3 for an ideal monatomic gas. The interpretation based on a 1D linear MHD model suggests that the thermal conductivity is suppressed by at least a factor of 3 in the hot flare loop at 9 MK and above. The viscosity coefficient is determined by coronal seismology from the observed wave when only considering the compressive viscosity dissipation. We find that to interpret the rapid wave damping, the classical compressive viscosity coefficient needs to be enhanced by a factor of 15 as the upper limit.

Authors: Tongjiang Wang, Leon Ofman, Xudong Sun, Elena Provornikova, Joseph M. Davila
Projects: SDO-AIA

Publication Status: ApJ Letter accepted, 2015
Last Modified: 2015-09-09 09:48
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Validation of Spherically Symmetric Inversion by Use of a Tomographic Reconstructed Three-Dimensional Electron Density of the Solar Corona  

Tongjiang Wang   Submitted: 2014-05-12 18:33

Determination of the coronal electron density by the inversion of white-light polarized brightness (pB) measurements by coronagraphs is a classic problem in solar physics. An inversion technique based on the spherically symmetric geometry (Spherically Symmetric Inversion, SSI) was developed in the 1950s, and has been widely applied to interpret various observations. However, to date there is no study about uncertainty estimation of this method. In this study we present the detailed assessment of this method using a three-dimensional (3D) electron density in the corona from 1.5 to 4 Rsun as a model, which is reconstructed by tomography method from STEREO/COR1 observations during solar minimum in February 2008. We first show in theory and observation that the spherically symmetric polynomial approximation (SSPA) method and the Van de Hulst inversion technique are equivalent. Then we assess the SSPA method using synthesized pB images from the 3D density model, and find that the SSPA density values are close to the model inputs for the streamer core near the plane of the sky (POS) with differences generally less than a factor of two or so; the former has the lower peak but more spread in both longitudinal and latitudinal directions than the latter. We estimate that the SSPA method may resolve the coronal density structure near the POS with angular resolution in longitude of about 50 degrees. Our results confirm the suggestion that the SSI method is applicable to the solar minimum streamer (belt) as stated in some previous studies. In addition, we demonstrate that the SSPA method can be used to reconstruct the 3D coronal density, roughly in agreement with that by tomography for a period of low solar activity. We suggest that the SSI method is complementary to the 3D tomographic technique in some cases, given that the development of the latter is still an ongoing research effort.

Authors: Tongjiang Wang and Joseph M. Devila
Projects: STEREO

Publication Status: Accepted by Solar Physics, May 12 2014.
Last Modified: 2014-05-14 13:09
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Three-dimensional MHD modeling of propagating disturbances in fan-like coronal loops  

Tongjiang Wang   Submitted: 2013-08-13 10:30

Quasi-periodic propagating intensity disturbances (PDs) have been observed in large coronal loops in EUV images over a decade, and are widely accepted to be slow magnetosonic waves. However, spectroscopic observations from Hinode/EIS revealed their association with persistent coronal upflows, making this interpretation debatable. Motivated by the scenario that the coronal upflows could be cumulative result of numerous individual flow pulses generated by sporadic heating events (nanoflares) at the loop base, we construct a velocity driver with repetitive tiny pulses, whose energy frequency distribution follows the flare power-law scaling. We then perform 3D MHD modeling of an idealized bipolar active region by applying this broadband velocity driver at the footpoints of large coronal loops which appear open in the computational domain. Our model successfully reproduces the PDs with similar features as the observed, and shows that any upflow pulses inevitably excite slow magnetosonic wave disturbances propagating along the loop. We find that the generated PDs are dominated by the wave signature as their propagation speeds are consistent with the wave speed in the presence of flows, and the injected flows rapidly decelerate with height. Our simulation results suggest that the observed PDs and associated persistent upflows may be produced by small-scale impulsive heating events (nanoflares) at the loop base, and that the flows and waves may both contribute to the PDs at lower heights.

Authors: Tongjiang Wang, Leon Ofman, and Joseph M. Davila
Projects: Hinode/EIS,SDO-AIA

Publication Status: Accepted by ApJ Letters in August 2013
Last Modified: 2013-08-13 11:02
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Growing transverse oscillations of a multistranded loop observed by SDO/AIA  

Tongjiang Wang   Submitted: 2012-04-09 18:38

The first evidence of transverse oscillations of a multistranded loop with growing amplitudes and internal coupling observed by the Atomspheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. The loop oscillation event occurred on 2011 March 8, triggered by a CME. The multiwavelength analysis reveals the presence of multithermal strands in the oscillating loop, whose dynamic behaviors are temperature-dependent, showing differences in their oscillation amplitudes, phases and emission evolution. The physical parameters of growing oscillations of two strands in 171 Å are measured and the 3-D loop geometry is determined using STEREO-A/EUVI data. These strands have very similar frequencies, and between two 193 Å strands a quarter-period phase delay sets up. These features suggest the coupling between kink oscillations of neighboring strands and the interpretation by the collective kink mode as predicted by some models. However, the temperature dependence of the multistarnded loop oscillations was not studied previously and needs further investigation. The transverse loop oscillations are associated with intensity and loop width variations. We suggest that the amplitude-growing kink oscillations may be a result of continuous non-periodic driving by magnetic deformation of the CME, which deposits energy into the loop system at a rate faster than its loss

Authors: Tongjiang Wang, Leon Ofman, Joseph M. Davila, Yang Su
Projects: SDO-AIA

Publication Status: Accepted by ApJ Letter (2012)
Last Modified: 2012-04-10 11:29
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Underflight calibration of SOHO/CDS and Hinode/EIS with EUNIS-07  

Tongjiang Wang   Submitted: 2011-09-30 09:49

Flights of Goddard Space Flight Center's Extreme-Ultraviolet Normal-IncidenceSpectrograph (EUNIS) sounding rocket in 2006 and 2007 provided updatedradiometric calibrations for SOHO/CDS and Hinode/EIS. EUNIS carried twoindependent imaging spectrographs covering wavebands of 300-370 A in firstorder and 170-205 A in second order. After each flight, end-to-end radiometriccalibrations of the rocket payload were carried out in the same facility usedfor pre-launch calibrations of CDS and EIS. During the 2007 flight, EUNIS, SOHOCDS and Hinode EIS observed the same solar locations, allowing the EUNIScalibrations to be directly applied to both CDS and EIS. The measured CDS NIS 1line intensities calibrated with the standard (version 4) responsivities withthe standard long-term corrections are found to be too low by a factor of 1.5due to the decrease in responsivity. The EIS calibration update is performed intwo ways. One is using the direct calibration transfer of the calibratedEUNIS-07 short wavelength (SW) channel. The other is using the insensitive linepairs, in which one member was observed by EUNIS-07 long wavelength (LW)channel and the other by EIS in either LW or SW waveband. Measurements fromboth methods are in good agreement, and confirm (within the measurementuncertainties) the EIS responsivity measured directly before the instrument'slaunch. The measurements also suggest that the EIS responsivity decreased by afactor of about 1.2 after the first year of operation. The shape of the EIS SWresponse curve obtained by EUNIS-07 is consistent with the one measured inlaboratory prior to launch. The absolute value of the quiet-Sun He II 304 Aintensity measured by EUNIS-07 is consistent with the radiance measured by CDSNIS in quiet regions near the disk center and the solar minimum irradianceobtained by CDS NIS and SDO/EVE recently.

Authors: Tongjiang Wang, Roger J. Thomas, Jeffrey W. Brosius, Peter R. Young, Douglas M. Rabin, Joseph M. Davila, Giulio Del Zanna
Projects: EUNIS

Publication Status: accepted by ApJ Supplement, publication in December 2011, V197-2 issue
Last Modified: 2011-09-30 10:49
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Propagating disturbances in coronal loops: flows or waves?  

Tongjiang Wang   Submitted: 2011-02-08 10:54

Quasi-periodic intensity disturbances propagating upward along the coronal structure have been extensively studied using EUV imaging observations from SOHO/EIT and TRACE. They were interpreted as either slow mode magnetoacoustic waves or intermittent upflows. In this study we aim at demonstrating that time series of spectroscopic observations are critical to solve this puzzle. Propagating intensity and Doppler shift disturbances in fanlike coronal loops are analyzed in multiple wavelengths using the sit-and-stare observations from Hinode/EIS. We find that the disturbances did not cause the blue-wing asymmetry of spectral profiles in the warm (~1.5 MK) coronal lines. The estimated small line-of-sight velocities also did not support the intermittent upflow interpretation. In the hot (~2 MK) coronal lines the disturbances did cause the blue-wing asymmetry, but the double fits revealed that a high-velocity minor component is steady and persistent, while the propagating intensity and Doppler shift disturbances are mainly due to variations of the core component, therefore, supporting the slow wave interpretation. However, the cause for blueward line asymmetries remains unclear.

Authors: T.J. Wang, L. Ofman, and J. M. Davila
Projects: Hinode/EIS

Publication Status: Hinode 4 meeting (2010), submitted (ASP Conference Series)
Last Modified: 2011-02-09 09:16
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Standing Slow-Mode Waves in Hot Coronal Loops: Observations, Modeling, and Coronal Seismology (Invited Review)  

Tongjiang Wang   Submitted: 2010-11-11 21:35

Strongly damped Doppler shift oscillations are observed frequently associated with flarelike events in hot coronal loops. In this paper, a review of the observed properties and the theoretical modeling is presented. Statistical measurements of physical parameters (period, decay time, and amplitude) have been obtained based on a large number of events observed by SOHO/SUMER and Yohkoh/BCS. Several pieces of evidence are found to support their interpretation in terms of the fundamental standing longitudinal slow mode. The high excitation rate of these oscillations in small- or micro-flares suggest that the slow mode waves are a natural response of the coronal plasma to impulsive heating in closed magnetic structure. The strong damping and the rapid excitation of the observed waves are two major aspects of the waves that are poorly understood, and are the main subject of theoretical modeling. The slow waves are found mainly damped by thermal conduction and viscosity in hot coronal loops. The mode coupling seems to play an important role in rapid excitation of the standing slow mode. Several seismology applications such as determination of the magnetic field, temperature, and density in coronal loops are demonstrated. Further, some open issues are discussed.

Authors: Tongjiang Wang
Projects: SoHO-SUMER

Publication Status: accepted to Space Science Review
Last Modified: 2010-11-12 08:18
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Absolute radiometric calibration of the EUNIS-06 170-205 A channel and calibration update for CDS/NIS  

Tongjiang Wang   Submitted: 2009-12-11 15:04

The Extreme-Ultraviolet Normal-Incidence Spectrograph sounding-rocket payload was flown on 2006 April 12 (EUNIS-06), carrying two independent imaging spectrographs covering wave bands of 300-370 A in first order and 170-205 A in second order, respectively. The absolute radiometric response of the EUNIS-06 long-wavelength (LW) channel was directly measured in the same facility used to calibrate CDS prior to the SOHO launch. Because the absolute calibration of the short-wavelength (SW) channel could not be obtained from the same lab configuration, we here present a technique to derive it using a combination of solar LW spectra and density- and temperature-insensitive line intensity ratios. The first step in this procedure is to use the coordinated, cospatial EUNIS and SOHO/CDS spectra to carry out an intensity calibration update for the CDS NIS-1 waveband, which shows that its efficiency has decreased by a factor about 1.7 compared to that of the previously implemented calibration. Then, theoretical insensitive line ratios obtained from CHIANTI allow us to determine absolute intensities of emission lines within the EUNIS SW bandpass from those of cospatial CDS/NIS-1 spectra after the EUNIS LW calibration correction. A total of 12 ratios derived from intensities of 5 CDS and 12 SW emission lines from Fe Fe X - Fe XIII yield an instrumental response curve for the EUNIS-06 SW channel that matches well to a relative calibration which relied on combining measurements of individual optical components. Taking into account all potential sources of error, we estimate that the EUNIS-06 SW absolute calibration is accurate to about 20%.

Authors: Tongjiang Wang, Jeffrey W. Brosius, Roger J. Thomas, Douglas M. Rabin, Joseph M. Davila
Projects: EUNIS

Publication Status: ApJ Suppl. 2009, accepted
Last Modified: 2009-12-14 09:29
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Hinode/EIS observations of propagating low-frequency slow magnetoacoustic waves in fan-like coronal loops  

Tongjiang Wang   Submitted: 2009-08-03 12:43

We report the first observation of multiple-periodic propagating disturbances along a fan-like coronal structure simultaneously detected in both intensity and Doppler shift in the Fe XII 195 A line with the EUV Imaging Spectrometer (EIS) onboard Hinode. A new application of coronal seismology is provided based on this observation. We analyzed the EIS sit-and-stare mode observation of oscillations using the running difference and wavelet techniques. Two harmonics with periods of 12 and 25 min are detected. We measured the Doppler shift amplitude of 1-2 km s-1, the relative intensity amplitude of 3%-5% and the apparent propagation speed of 100-120 km s-1. The amplitude relationship between intensity and Doppler shift oscillations provides convincing evidence that these propagating features are a manifestation of slow magnetoacoustic waves. Detection lengths (over which the waves are visible) of the 25 min wave are about 70-90 Mm, much longer than those of the 5 min wave previously detected by TRACE. This difference may be explained by the dependence of damping length on the wave period for thermal conduction. Based on a linear wave theory, we derive an inclination of the magnetic field to the line-of-sight about 59pm8 deg, a true propagation speed of 128pm25 km s-1 and a temperature of 0.7pm0.3 MK near the loop's footpoint from our measurements.

Authors: T. J. Wang, L. Ofman, J. M. Davila, J. T. Mariska
Projects: Hinode/EIS,TRACE

Publication Status: A&A Letter, 2009, in press
Last Modified: 2009-08-04 08:07
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Propagating slow magnetoacoustic waves in coronal loops observed by Hinode/EIS  

Tongjiang Wang   Submitted: 2009-02-25 16:09

We present the first Hinode/EIS observations of 5 min quasi-periodic oscillations detected in a transition-region line (He II) and five coronal lines (Fe X, Fe XII, Fe XIII, Fe XIV, and Fe XV) at the footpoint of a coronal loop. The oscillations exist throughout the whole observation, characterized by a series of wave packets with nearly constant period, typically persisting for 4-6 cycles with a lifetime of 20-30 min. There is an approximate in-phase relation between Doppler shift and intensity oscillations. This provides evidence for slow magnetoacoustic waves propagating upwards from the transition region into the corona. We find that the oscillations detected in the five coronal lines are highly correlated, and the amplitude decreases with increasing temperature. The amplitude of Doppler shift oscillations decrease by a factor of about 3, while that of relative intensity decreases by a factor of about 4 from Fe X to Fe XV. These oscillations may be caused by the leakage of the photospheric p-modes through the chromosphere and transition region into the corona, which has been suggested as the source for intensity oscillations previously observed by TRACE. The temperature dependence of the oscillation amplitudes can be explained by damping of the waves traveling along the loop with multithread structure near the footpoint. Thus, this property may have potential value for coronal seismology in diagnostic of temperature structure in a coronal loop.

Authors: T. J. Wang, L. Ofman, and J. M. Davila
Projects: Hinode/EIS

Publication Status: ApJ, May 2009 - v696 issue, (in press)
Last Modified: 2009-03-11 00:19
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Identification of different types of kink modes in coronal loops: principles and application to TRACE results  

Tongjiang Wang   Submitted: 2008-08-06 08:21

We explore the possible observational signatures of different types of kink modes (horizontal and vertical oscillations in their fundamental mode and second harmonic) that may arise in coronal loops, with the aim of determining how well the individual modes can be uniquely identified from time series of images. A simple, purely geometrical model is constructed to describe the different types of kink-mode oscillations. These are then `observed' from a given direction. In particular, we employ the 3D geometrical parameters of 14 TRACE loops of transverse oscillations to try to identify the correct observed wave mode. We find that for many combinations of viewing and loop geometry it is not straightforward to distinguish between at least two types of kink modes just using time series of images. We also considered Doppler signatures and find that these can help obtain unique identifications of the oscillation modes when employed in combination with imaging. We then compare the modeled spatial signatures with the observations of 14 TRACE loops. We find that out of three oscillations previously identified as fundamental horizontal mode oscillations, two cases appear to be fundamental vertical mode oscillations (but possibly combined with the fundamental horizontal mode), and one case appears to be a combination of the fundamental vertical and horizontal modes, while in three cases it is not possible to clearly distinguish between the fundamental mode and the second-harmonic of the horizontal oscillation. In five other cases it is not possible to clearly distinguish between a fundamental horizontal mode and the second-harmonic of a vertical mode.

Authors: T. J. Wang, S. K. Solanki, M. Selwa
Projects: TRACE

Publication Status: A&A, in press
Last Modified: 2008-09-23 20:55
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Direct observation of high-speed plasma outflows produced by magnetic reconnection in solar impulsive events  

Tongjiang Wang   Submitted: 2007-04-25 21:55

Spectroscopic observations of a solar limb flare recorded by SUMER on SOHO reveal, for the first time, hot fast magnetic reconnection outflows in the corona. As the reconnection site rises across the SUMER spectrometer slit, significant blue- and red-shift signatures are observed in sequence in the Fe XIX line, reflecting upflows and downflows of hot plasma jets, respectively. With the projection effect corrected, the measured outflow speed is between about 900-3500 km s-1, consistent with theoretical predictions of the Alfvénic outflows in magnetic reconnection region in solar impulsive events. Based on theoretic models, the magnetic field strength near the reconnection region is estimated to be 19-37 Gauss.

Authors: TONGJIANG WANG, LINHUI SUI and JIONG QIU
Projects: SoHO-SUMER,TRACE,RHESSI

Publication Status: ApJ Lett. Accepted
Last Modified: 2007-04-26 08:05
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Determination of the Coronal Magnetic Field by Hot Loop Oscillations Observed by SUMER and SXT  

Tongjiang Wang   Submitted: 2006-10-30 21:29

We apply a new method to determine the magnetic field in coronal loops using observations of coronal loop oscillations. We analyze seven Doppler shift oscillation events detected by SUMER in the hot flare line Fe XIX to obtain oscillation periods of these events. The geometry, temperature, and electron density of the oscillating loops are measured from coordinated multi-channel soft X-ray imaging observations from SXT. All the oscillations are consistent with standing slow waves in their fundamental mode. The parameters are used to calculate the magnetic field of coronal loops based on MHD wave theory. For the seven events, the plasma eta is in the range 0.15-0.91 with a mean of 0.33pm0.26, and the estimated magnetic field varies between 21-61 G with a mean of 34pm14 G. With background emission subtracted, the estimated magnetic field is reduced by 9%-35%. The maximum backgroud subtraction gives a mean of 22pm13 G in the range 12-51 G. We discuss measurement uncertainties and the prospect of determining coronal loop magnetic fields from future observations of coronal loops and Doppler shift oscillations.

Authors: Tongjiang Wang, Davina E. Innes, and Jiong Qiu
Projects: SoHO-SUMER

Publication Status: ApJ, V656, No. 1, Feb 10 2007 issue
Last Modified: 2006-10-31 12:06
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Fe XIX observations of active region brightenings in the corona  

Tongjiang Wang   Submitted: 2006-05-22 11:05

Small flarelike brightenings seen in the hot flare line, FeXIX, by the spectrometer SUMER on SOHO are analysed. We observe active region coronae about 30 Mm off the limb of the Sun for a period of several days. Brightenings are observed with a frequency 3-14 per hour and their lifetimes range from 5-150 min, with an average of about 25 min. The measured size of the events along the spectrometer slit range from 2-67 Mm, but most are around 7 Mm. Like soft X-ray active region transient brightenings, they range in estimated thermal energy from 1026 to 1029 ergs with a power law index of 1.7 to 1.8, beyond 1027 ergs. We conclude that they are the coronal parts of loops heated to > 6 MK by soft X-ray microflares.

Authors: T. J. Wang, D.E. Innes, and S.K. Solanki
Projects: SoHO-SUMER

Publication Status: A&A, 2006, in press
Last Modified: 2006-05-24 09:48
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Initiation of hot coronal loop oscillations: spectral features  

Tongjiang Wang   Submitted: 2005-03-02 10:58

We explore the excitation of hot loop oscillations observed with the SUMER spectrograph on SOHO by analysing FeXIX and FeXXI spectral line profiles in the initial phase of the events. We investigate all 54 Doppler shift oscillations in 27 flare-like events, whose physical parameters have been measured so far. In nearly 50% of the cases, the spectral evolution reveals the presence of two spectral components, one of them almost undisturbed, the other highly shifted. We find that the shifted component reaches maximum Doppler shift (on the order of 100-300 km s-1 and peak intensity almost simultaneously. The velocity amplitude of the shifted component has no correlation with the oscillation amplitudes. These features imply that in these events the initial shifts are not caused by the locally oscillating plasma (or waves), but most likely by a pulse of hot plasma travelling along the loop through the slit position. This interpretation is also supported by several examples showing that standing slow mode waves are set up immediately after the initial line shift pulse (standing slow mode waves are inferred from the 1/4-period phase relationship between the velocity and intensity oscillations). We re-measure the physical parameters of the 54 Doppler oscillations by fitting the time profiles excluding the first peak, and find that the periods are almost unchanged, damping times are shorter by 5%, and amplitudes are smaller by 37% than measured when the first peak is included. We also measure the velocity of the net (background) flow during the oscillations, which is found to be nearly zero. Our result of initial hot flows supports the model of single footpoint (asymmetric) excitation, but contradicts chromospheric evaporation as the trigger.

Authors: T. J. Wang, S. K. Solanki, D. E. Innes, and W. Curdt
Projects: None

Publication Status: A&A, 2005, in press
Last Modified: 2005-03-02 10:58
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Vertical oscillations of a coronal loop observed by TRACE  

Tongjiang Wang   Submitted: 2004-06-27 05:19

We report on a loop oscillation event observed by TRACE in the 195 Å bandpass at the solar limb. The difference images reveal the first evidence for vertical kink oscillations of the loop, i.e., alternately expanding and shrinking motions, in contrast to horizontal transverse loop oscillations reported before, which exhibit swaying motions. Based on the 3D geometry of the oscillating loop derived from the observation by fitting with a circular or elliptical loop model, we simulate these two kinds of global kink modes and find that only the vertical oscillations produce a signature in the difference images in agreement with the observations. We also find that the oscillating loop is associated with intensity variations. Based on the measured displacement amplitude, the simulation predicts an intensity variation of about 13% due to density changes produced by the change of the loop length. The observed intensity changes have the same sign but are considerably larger than the predictions although the error bars are also large. This suggests that these oscillations are compressible.

Authors: T. J. Wang and S. K. Solanki
Projects: None,TRACE

Publication Status: A&A 421, L33-L36 (2004)
Last Modified: 2004-06-27 05:19
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Abstracts by Author
Effect of transport coefficients on excitation of flare-induced standing slow-mode waves in coronal loops
Variation of Coronal Activity from the Minimum to Maximum of Solar Cycle 24 using Three Dimensional Coronal Electron Density Reconstructions from STEREO/COR1
Waves in Solar Coronal Loops
Evidence of thermal conduction suppression in hot coronal loops: Supplementary results
Evidence of thermal conduction suppression in a solar flaring loop by coronal seismology of slow-mode waves
Validation of Spherically Symmetric Inversion by Use of a Tomographic Reconstructed Three-Dimensional Electron Density of the Solar Corona
Three-dimensional MHD modeling of propagating disturbances in fan-like coronal loops
Growing transverse oscillations of a multistranded loop observed by SDO/AIA
Underflight calibration of SOHO/CDS and Hinode/EIS with EUNIS-07
Propagating disturbances in coronal loops: flows or waves?
Standing Slow-Mode Waves in Hot Coronal Loops: Observations, Modeling, and Coronal Seismology (Invited Review)
Absolute radiometric calibration of the EUNIS-06 170-205 A channel and calibration update for CDS/NIS
Hinode/EIS observations of propagating low-frequency slow magnetoacoustic waves in fan-like coronal loops
Propagating slow magnetoacoustic waves in coronal loops observed by Hinode/EIS
Identification of different types of kink modes in coronal loops: principles and application to TRACE results
Direct observation of high-speed plasma outflows produced by magnetic reconnection in solar impulsive events
Determination of the Coronal Magnetic Field by Hot Loop Oscillations Observed by SUMER and SXT
Fe XIX observations of active region brightenings in the corona
Initiation of hot coronal loop oscillations: spectral features
Vertical oscillations of a coronal loop observed by TRACE
CORONAL LOOP OSCILLATIONS: OVERVIEW OF RECENT RESULTS
Hot coronal loop oscillations observed with SUMER: examples and statistics (A full paper)
Slow-mode standing waves observed by SUMER in hot coronal loops
Initial features of an X-class flare (21 April 2002) observed with SUMER and TRACE
Hot loop oscillations seen by SUMER: examples and statistics
Emergence and drastic break-down of a twisted flux rope to trigger strong solar flares in the active region NOAA 9026
Doppler-shift Oscillations of Hot Solar Coronal Plasma seen by SUMER: a Signature of Loop Oscillations
OSCILLATING HOT LOOPS OBSERVED BY SUMER
The large-scale coronal field structure and source-region features for a halo CME

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

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