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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

A new facility for airborne solar astronomy: NASA's WB-57 at the 2017 total solar eclipse  

Amir Caspi   Submitted: 2020-06-09 12:37

NASA's WB-57 High Altitude Research Program provides a deployable, mobile, stratospheric platform for scientific research. Airborne platforms are of particular value for making coronal observations during total solar eclipses because of their ability both to follow the Moon's shadow and to get above most of the atmospheric airmass that can interfere with astronomical observations. We used the 2017 Aug 21 eclipse as a pathfinding mission for high-altitude airborne solar astronomy, using the existing high-speed visible-light and near-/mid-wave infrared imaging suite mounted in the WB-57 nose cone. In this paper, we describe the aircraft, the instrument, and the 2017 mission; operations and data acquisition; and preliminary analysis of data quality from the existing instrument suite. We describe benefits and technical limitations of this platform for solar and other astronomical observations. We present a preliminary analysis of the visible-light data quality and discuss the limiting factors that must be overcome with future instrumentation. We conclude with a discussion of lessons learned from this pathfinding mission and prospects for future research at upcoming eclipses, as well as an evaluation of the capabilities of the WB-57 platform for future solar astronomy and general astronomical observation.

Authors: Amir Caspi, Daniel B. Seaton, Constantine C. C. Tsang, Craig E. DeForest, Paul Bryans, Edward E. DeLuca, Steven Tomczyk, Joan T. Burkepile, Thomas 'Tony' Casey, John Collier, Donald 'DD' Darrow, Dominic Del Rosso, Daniel D. Durda, Peter T. Gallagher, Leon Golub, Matthew Jacyna, David 'DJ' Johnson, Philip G. Judge, Cary 'Diddle' Klemm, Glenn T. Laurent, Johanna Lewis, Charles J. Mallini, Thomas 'Duster' Parent, Timothy Propp, Andrew J. Steffl, Jeff Warner, Matthew J. West, John Wiseman, Mallory Yates, Andrei N. Zhukov, and the NASA WB-57 2017 Eclipse Observing Team
Projects: MLSO/CoMP,GOES/SUVI,Other

Publication Status: Published -- Caspi et al. 2020, ApJ, 895, 131; DOI: 10.3847/1538-4357/ab89a8
Last Modified: 2020-06-10 08:53
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MinXSS-2 CubeSat mission overview: Improvements from the successful MinXSS-1 mission  

Amir Caspi   Submitted: 2020-06-09 12:32

The second Miniature X-ray Solar Spectrometer (MinXSS-2) CubeSat, which begins its flight in late 2018, builds on the success of MinXSS-1, which flew from 2016-05-16 to 2017-05-06. The science instrument is more advanced – now capable of greater dynamic range with higher energy resolution. More data will be captured on the ground than was possible with MinXSS-1 thanks to a sun-synchronous, polar orbit and technical improvements to both the spacecraft and the ground network. Additionally, a new open-source beacon decoder for amateur radio operators is available that can automatically forward any captured MinXSS data to the operations and science team. While MinXSS-1 was only able to downlink about 1 MB of data per day corresponding to a data capture rate of about 1%, MinXSS-2 will increase that by at least a factor of 6. This increase of data capture rate in combination with the mission’s longer orbital lifetime will be used to address new science questions focused on how coronal soft X-rays vary over solar cycle timescales and what impact those variations have on the earth’s upper atmosphere.

Authors: James Paul Mason, Thomas N. Woods, Phillip C. Chamberlin, Andrew Jones, Rick Kohnert, Bennet Schwab, Robert Sewell, Amir Caspi, Christopher S. Moore, Scott Palo, Stanley C. Solomon, Harry Warren
Projects: MinXSS

Publication Status: Published -- Mason et al. 2020, Adv. Space Res., 66, 3; DOI: 10.1016/j.asr.2019.02.011
Last Modified: 2020-06-10 08:54
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New Solar Irradiance Measurements from the Miniature X-Ray Solar Spectrometer Cubesat  

Amir Caspi   Submitted: 2019-08-26 13:09

The goal of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is to explore the energy distribution of soft X-ray (SXR) emissions from the quiescent Sun, active regions, and during solar flares and to model the impact on Earthʼs ionosphere and thermosphere. The energy emitted in the SXR range (0.1-10 keV) can vary by more than a factor of 100, yet we have limited spectral measurements in the SXRs to accurately quantify the spectral dependence of this variability. The MinXSS primary science instrument is an Amptek, Inc. X123 X-ray spectrometer that has an energy range of 0.5-30 keV with a nominal 0.15 keV energy resolution. Two flight models have been built. The first, MinXSS-1, has been making science observations since 2016 June 9 and has observed numerous flares, including more than 40 C-class and 7 M-class flares. These SXR spectral measurements have advantages over broadband SXR observations, such as providing the capability to derive multiple-temperature components and elemental abundances of coronal plasma, improved irradiance accuracy, and higher resolution spectral irradiance as input to planetary ionosphere simulations. MinXSS spectra obtained during the M5.0 flare on 2016 July 23 highlight these advantages and indicate how the elemental abundance appears to change from primarily coronal to more photospheric during the flare. MinXSS-1 observations are compared to the Geostationary Operational Environmental Satellite (GOES) X-ray Sensor (XRS) measurements of SXR irradiance and estimated corona temperature. Additionally, a suggested improvement to the calibration of the GOES XRS data is presented.

Authors: Thomas N. Woods, Amir Caspi, Phillip C. Chamberlin, Andrew Jones, Richard Kohnert, James P. Mason, Christopher S. Moore, Scott Palo, Colden Rouleau, Stanley C. Solomon, Janet Machol, Rodney Viereck
Projects: GOES X-rays,MinXSS

Publication Status: Published -- Woods et al. 2017, ApJ, 835, 122; DOI: 10.3847/1538-4357/835/2/122
Last Modified: 2019-08-27 10:10
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MinXSS-1 CubeSat On-Orbit Pointing and Power Performance: The First Flight of the Blue Canyon Technologies XACT 3-axis Attitude Determination and Control System  

Amir Caspi   Submitted: 2019-08-26 13:02

The Miniature X-ray Solar Spectrometer (MinXSS) is a three-unit (3U) CubeSat designed for a three-month mission to study solar soft X-ray spectral irradiance. The first of the two flight models was deployed from the International Space Station in May 2016, and operated for one year before its natural deorbiting. This was the first flight of the Blue Canyon Technologies XACT 3-axis attitude determination and control system - a commercially available, high-precision pointingsystem. The performance of the pointing system on orbit was characterized, including performance at low altitudes where drag torque builds up. It was found that the pointing accuracy was 0.0042° - 0.0117° (15" - 42", 3 σ, axis dependent) consistently from 190 km - 410 km, slightly better than the specification sheet states. Peak-to-peak jitter was estimated to be 0.0073° (10 s-1) - 0.0183° (10 s-1) (26" (10 s-1) - 66" (10 s-1), 3 σ). The system was capable of dumping momentum until an altitude of 185 km. Small amounts of sensor degradation were found in the star tracker and coarse sun sensor. The mission profile did not require high-agility maneuvers, so it was not possible to characterize this metric. Without a GPS receiver, it was necessary to periodically upload ephemeris information to update the orbit propagation model and maintain pointing. At 400 km, these uploads were required once every other week; at ∼270 km, they were required every day. The power performance of the electric power system was also characterized, including use of a novel pseudo-peak power tracker - a resistor that limited the current draw from the battery on the solar panels. With 19 30% efficient solar cells and an 8 W system load, the power balance had 65% of margin on orbit. The current paper presents several recommendations to other CubeSat programs throughout.

Authors: James P. Mason, Matt Baumgart, Bryan Rogler, Chloe Downs, Margaret Williams, Thomas N. Woods, Scott Palo, Phillip C. Chamberlin, Stanley Solomon, Andrew Jones, Xinlin Li, Rick Kohnert, Amir Caspi
Projects: MinXSS

Publication Status: Published -- Mason et al. 2017, J. Small Satellites, 6(3), 651
Last Modified: 2019-08-27 10:10
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The Multi-instrument (EVE-RHESSI) DEM for Solar Flares, and Implications for Nonthermal Emission  

Amir Caspi   Submitted: 2019-08-26 12:53

Solar flare X-ray spectra are typically dominated by thermal bremsstrahlung emission in the soft X-ray (<~10 keV) energy range; for hard X-ray energies (≳30 keV), emission is typically nonthermal from beams of electrons. The low-energy extent of nonthermal emission has only been loosely quantified. It has been difficult to obtain a lower limit for a possible nonthermal cutoff energy due to the significantly dominant thermal emission. Here we use solar flare data from the extreme ultraviolet Variability Experiment on board the Solar Dynamics Observatory and X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager to calculate the Differential Emission Measure (DEM). This improvement over the isothermal approximation and any single-instrument DEM helps to resolve ambiguities in the range where thermal and nonthermal emission overlap, and to provide constraints on the low-energy cutoff. In the model, thermal emission is from a DEM that is parameterized as multiple Gaussians in Log(T). Nonthermal emission results from a photon spectrum obtained using a thick-target emission model. Spectra for both instruments are fit simultaneously in a self-consistent manner. Our results have been obtained using a sample of 52 large (Geostationary Operational Environmental Satellite X- and M-class) solar flares observed between 2011 and 2013. It turns out that it is often possible to determine low-energy cutoffs early (in the first two minutes) during large flares. Cutoff energies at these times are typically low, less than 10 keV, when assuming coronal abundances. With photospheric abundances, cutoff energies are typically ∼10 keV higher, in the ∼17-25 keV range.

Authors: James M. McTiernan, Amir Caspi, Harry P. Warren
Projects: GOES X-rays,RHESSI,SDO-EVE

Publication Status: Published -- McTiernan, J. M., Caspi, A., & Warren, H. P. 2019, ApJ, 881, 161; DOI 10.3847/1538-4357/ab2fcc
Last Modified: 2019-08-27 10:10
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The Instruments and Capabilities of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSats  

Amir Caspi   Submitted: 2018-01-24 16:01

The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is the first solar science oriented CubeSat mission flown for the NASA Science Mission Directorate, with the main objective of measuring the solar soft X-ray (SXR) flux and a science goal of determining its influence on Earth's ionosphere and thermosphere. These observations can also be used to investigate solar quiescent, active region, and flare properties. The MinXSS X-ray instruments consist of a spectrometer, called X123, with a nominal 0.15 keV full-width-half-maximum (FWHM) resolution at 5.9 keV and a broadband X-ray photometer, called XP. Both instruments are designed to obtain measurements from 0.5-30 keV at a nominal time cadence of 10 seconds. A description of the MinXSS instruments, performance capabilities, and relation to the Geostationary Operational Environmental Satellite (GOES) 0.1-0.8 nm flux are discussed in this article. Early MinXSS results demonstrate the capability to measure variations of the solar spectral SXR flux between 0.8-12 keV from at least GOES A5-M5 (5x10-8-5x10-5 W m-2) levels and infer physical properties (temperature and emission measure) from the MinXSS data alone. Moreover, coronal elemental abundances can be inferred, specifically Fe, Ca, Si, Mg, S, Ar, and Ni, when there is sufficiently high count rate at each elemental spectral feature. Additionally, temperature response curves and emission measure loci demonstrate the MinXSS sensitivity to plasma emission at different temperatures. MinXSS observations coupled with those from other solar observatories can help address some of the most compelling questions in solar coronal physics. Finally, simultaneous observations by MinXSS and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) can provide the most spectrally complete soft X-ray solar flare photon flux measurements to date.

Authors: Christopher S. Moore, Amir Caspi, Thomas N. Woods, Phillip C. Chamberlin, Brian R. Dennis, Andrew R. Jones, James P. Mason, Richard A. Schwartz, Anne K. Tolbert
Projects: GOES X-rays,MinXSS,RHESSI

Publication Status: Published -- Moore et al. 2018, Sol. Phys., 293, 21; DOI: 10.1007/s11207-018-1243-3
Last Modified: 2018-01-29 14:04
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First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events  

Amir Caspi   Submitted: 2017-11-22 16:58

We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite. While NuSTAR was designed as an astrophysics mission, it can observe the Sun above 2 keV with unprecedented sensitivity due to its pioneering use of focusing optics. NuSTAR first observed quiet Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun transient brightenings on time scales of 100 s and set upper limits on emission in two energy bands. We set 2.5-4 keV limits on brightenings with time scales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10-20 keV limits on brightenings with time scales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the NuSTAR sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background.

Authors: Andrew J. Marsh, David M. Smith, Lindsay Glesener, Iain G. Hannah, Brian W. Grefenstette, Amir Caspi, Säm Krucker, Hugh S. Hudson, Kristin K. Madsen, Stephen M. White, Matej Kuhar, Paul J. Wright, Steven E. Boggs, Finn E. Christensen, William W. Craig, Charles J. Hailey, Fiona A. Harrison, Daniel Stern, William W. Zhang
Projects: NuSTAR,RHESSI,Yohkoh-SXT

Publication Status: Published -- Marsh, A. J., et al. 2017, ApJ, 849, 131; DOI 10.3847/1538-4357/aa9122
Last Modified: 2017-11-25 10:03
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First flight of the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument  

Amir Caspi   Submitted: 2016-10-05 14:51

The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument is a balloon-borne telescope designed to study solar-flare particle acceleration and transport. We describe GRIPS's first Antarctic long-duration flight in January 2016 and report preliminary calibration and science results. Electron and ion dynamics, particle abundances and the ambient plasma conditions in solar flares can be understood by examining hard X-ray (HXR) and gamma-ray emission (20 keV to 10 MeV). Enhanced imaging, spectroscopy and polarimetry of flare emissions in this energy range are needed to study particle acceleration and transport questions. The GRIPS instrument is specifically designed to answer questions including: What causes the spatial separation between energetic electrons producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? GRIPS's key technological improvements over the current solar state of the art at HXR/gamma-ray energies, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), include 3D position-sensitive germanium detectors (3D-GeDs) and a single-grid modulation collimator, the multi-pitch rotating modulator (MPRM). The 3D-GeDs have spectral FWHM resolution of a few hundred keV and spatial resolution <1 mm^3. For photons that Compton scatter, usually ≳150 keV, the energy deposition sites can be tracked, providing polarization measurements as well as enhanced background reduction through Compton imaging. Each of GRIPS's detectors has 298 electrode strips read out with ASIC/FPGA electronics. In GRIPS's energy range, indirect imaging methods provide higher resolution than focusing optics or Compton imaging techniques. The MPRM gridimaging system has a single-grid design which provides twice the throughput of a bi-grid imaging system like RHESSI. The grid is composed of 2.5 cm deep tungsten-copper slats, and quasi-continuous FWHM angular coverage from 12.5-162 arcsecs are achieved by varying the slit pitch between 1-13 mm. This angular resolution is capable of imaging the separate magnetic loop footpoint emissions in a variety of flare sizes. In comparison, RHESSI's 35-arcsec resolution at similar energies makes the footpoints resolvable in only the largest flares.

Authors: Nicole Duncan, P. Saint-Hilaire, A. Y. Shih, G. J. Hurford, H. M. Bain, M. Amman, B. A. Mochizuki, J. Hoberman, J. Olson, B. A. Maruca, N. M. Godbole, D. M. Smith, J. Sample, N. A. Kelley, A. Zoglauer, A. Caspi, P. Kaufmann, S. Boggs, R. P. Lin
Projects: Other

Publication Status: Published -- Duncan et al. 2016, Proc. SPIE 9905, Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, 99052Q; DOI: 10.1117/12.2233859
Last Modified: 2016-10-12 12:18
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The Miniature X-ray Solar Spectrometer (MinXSS) CubeSats: spectrometer characterization techniques, spectrometer capabilities, and solar science objectives  

Amir Caspi   Submitted: 2016-08-22 13:34

The Miniature X-ray Solar Spectrometer (MinXSS) are twin 3U CubeSats. The first of the twin CubeSats (MinXSS-1) launched in December 2015 to the International Space Station for deployment in mid-2016. Both MinXSS CubeSats utilize a commercial off the shelf (COTS) X-ray spectrometer from Amptek to measure the solar irradiance from 0.5 to 30 keV with a nominal 0.15 keV FWHM spectral resolution at 5.9 keV, and a LASP-developed X-ray broadband photometer with similar spectral sensitivity. MinXSS design and development has involved over 40 graduate students supervised by professors and professionals at the University of Colorado at Boulder. The majority of previous solar soft X-ray measurements have been either at high spectral resolution with a narrow bandpass or spectrally integrating (broadband) photometers. MinXSS will conduct unique soft X-ray measurements with moderate spectral resolution over a relatively large energy range to study solar active region evolution, solar flares, and the effects of solar soft X-ray emission on Earth's ionosphere. This paper focuses on the X-ray spectrometer instrument characterization techniques involving radioactive X-ray sources and the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF). Spectrometer spectral response, spectral resolution, response linearity are discussed as well as future solar science objectives.

Authors: Christopher S. Moore, Thomas N. Woods, Amir Caspi, James P. Mason
Projects: MinXSS

Publication Status: Published -- Moore et al. 2016, Proc. SPIE 9905, Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, 990509; DOI: 10.1117/12.2231945
Last Modified: 2019-08-27 10:10
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Miniature X-ray Solar Spectrometer (MinXSS) - A Science-Oriented, University 3U CubeSat  

Amir Caspi   Submitted: 2016-08-17 14:55

The Miniature X-ray Solar Spectrometer (MinXSS) is a 3-Unit (3U) CubeSat developed at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder (CU). Over 40 students contributed to the project with professional mentorship and technical contributions from professors in the Aerospace Engineering Sciences Department at CU and from LASP scientists and engineers. The scientific objective of MinXSS is to study processes in the dynamic Sun, from quiet-Sun to solar flares, and to further understand how these changes in the Sun influence the Earth's atmosphere by providing unique spectral measurements of solar soft x-rays (SXRs). The enabling technology providing the advanced solar SXR spectral measurements is the Amptek X123, a commercial-off-the-shelf (COTS) silicon drift detector (SDD). The Amptek X123 has a low mass (~324 g after modification), modest power consumption (~2.50 W), and small volume (6.86 cm x 9.91 cm x 2.54 cm), making it ideal for a CubeSat. This paper provides an overview of the MinXSS mission: the science objectives, project history, subsystems, and lessons learned that can be useful for the small-satellite community.

Authors: James Paul Mason, Thomas N. Woods, Amir Caspi, Phillip Chamberlin, Christopher Moore, Andrew Jones, Rick Kohnert, Xinlin Li, Scott Palo, & Stanley Solomon
Projects: MinXSS

Publication Status: Published -- Mason et al. 2016, J. Spacecraft Rockets, 53, 328; DOI: 10.2514/1.A33351
Last Modified: 2019-08-27 10:10
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Soft X-ray irradiance measured by the Solar Aspect Monitor on the Solar Dynamic Observatory Extreme ultraviolet Variability Experiment  

Amir Caspi   Submitted: 2016-05-26 11:04

The Solar Aspect Monitor (SAM) is a pinhole camera on the Extreme-ultraviolet Variability Experiment (EVE) aboard the Solar Dynamics Observatory (SDO). SAM projects the solar disk onto the CCD through a metallic filter designed to allow only solar photons shortward of 7 nm to pass. Contamination from energetic particles and out-of-band irradiance is, however, significant in the SAM observations. We present a technique for isolating the 0.01-7 nm integrated irradiance from the SAM signal to produce the first results of broadband irradiance for the time period from May 2010 to May 2014. The results of this analysis agree with a similar data product from EVE's EUV SpectroPhotometer (ESP) to within 25%. We compare our results with measurements from the Student Nitric Oxide Explorer (SNOE) Solar X-ray Photometer (SXP) and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) Solar EUV Experiment (SEE) at similar levels of solar activity. We show that the full-disk SAM broadband results compare well to the other measurements of the 0.01-7 nm irradiance. We also explore SAM's capability toward resolving spatial contribution from regions of solar disk in irradiance and demonstrate this feature with a case study of several strong flares that erupted from active regions on March 11, 2011.

Authors: C. Y. Lin, S. M. Bailey, A. Jones, D. Woodraska, A. Caspi, T. N. Woods, F. G. Eparvier, S. R. Wieman, L. V. Didkovsky
Projects: SDO-EVE

Publication Status: Published -- C. Y. Lin, S. M. Bailey, A. Jones, et al. 2016, J. Geophys. Res.: Space Physics, 121, 3648; DOI: 10.1002/2015JA021726
Last Modified: 2016-05-27 12:37
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The First X-ray Imaging Spectroscopy of Quiescent Solar Active Regions with NuSTAR  

Amir Caspi   Submitted: 2016-03-28 12:08

We present the first observations of quiescent active regions (ARs) using the Nuclear Spectroscopic Telescope Array (NuSTAR), a focusing hard X-ray telescope capable of studying faint solar emission from high-temperature and non-thermal sources. We analyze the first directly imaged and spectrally resolved X-rays above 2 keV from non-flaring ARs, observed near the west limb on 2014 November 1. The NuSTAR X-ray images match bright features seen in extreme ultraviolet and soft X-rays. The NuSTAR imaging spectroscopy is consistent with isothermal emission of temperatures 3.1-4.4 MK and emission measures 1-8 x 1046 cm-3. We do not observe emission above 5 MK, but our short effective exposure times restrict the spectral dynamic range. With few counts above 6 keV, we can place constraints on the presence of an additional hotter component between 5 and 12 MK of ~1046 cm-3 and ~1043 cm-3, respectively, at least an order of magnitude stricter than previous limits. With longer duration observations and a weakening solar cycle (resulting in an increased livetime), future NuSTAR observations will have sensitivity to a wider range of temperatures as well as possible non-thermal emission.

Authors: Iain G. Hannah, Brian W. Grefenstette, David M. Smith, Lindsay Glesener, Säm Krucker, Hugh S. Hudson, Kristin K. Madsen, Andrew Marsh, Stephen M. White, Amir Caspi, Albert Y. Shih, Fiona A. Harrison, Daniel Stern, Steven E. Boggs, Finn E. Christensen, William W. Craig, Charles J. Hailey, William W. Zhang
Projects: Other

Publication Status: Published -- Hannah, I. G. et al. 2016, ApJL, 820, L4; DOI 10.3847/2041-8205/820/1/L14
Last Modified: 2016-03-30 20:56
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Flare Footpoint Regions and a Surge Observed by the Hinode/EUV Imaging Spectrometer (EIS), RHESSI, and SDO/AIA  

Amir Caspi   Submitted: 2015-10-28 16:05

The Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft observed flare footpoint regions coincident with a surge for a M3.7 flare observed on 25 September 2011 at N12 E33 in active region 11302. The flare was observed in spectral lines of O VI, Fe X, Fe XII, Fe XIV, Fe XV, Fe XVI, Fe XVII, Fe XXIII and Fe XXIV. The EIS observations were made coincident with hard X-ray bursts observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Overlays of the RHESSI images on the EIS raster images at different wavelengths show a spatial coincidence of features in the RHESSI images with the EIS upflow and downflow regions, as well as loop-top or near-loop-top regions. A complex array of phenomena was observed including multiple evaporation regions and the surge, which was also observed by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) telescopes. The slit of the EIS spectrometer covered several flare footpoint regions from which evaporative upflows in Fe XXIII and Fe XXIV lines were observed with Doppler speeds greater than 500 km s-1. For ions such as Fe XV both evaporative outflows (~200 km s-1) and downflows (~30-50 km s-1) were observed. Non-thermal motions from 120 to 300 km s-1 were measured in flare lines. In the surge, Doppler speeds are found from about 0 to over 250 km s-1 in lines from ions such as Fe XIV. The non-thermal motions could be due to multiple sources slightly Doppler-shifted from each other or turbulence in the evaporating plasma. We estimate the energetics of the hard X-ray burst and obtain a total flare energy in accelerated electrons of ≥7x1028 ergs. This is a lower limit because only an upper limit can be determined for the low energy cutoff to the electron spectrum. We find that detailed modeling of this event would require a multi-threaded model due to its complexity.

Authors: George A. Doschek, Harry P. Warren, Brian R. Dennis, Jeffrey W. Reep, Amir Caspi
Projects: Hinode/EIS,RHESSI,SDO-AIA

Publication Status: Published -- Doschek, G. A., Warren, H. P., Dennis, B. R., Reep, J. W., & Caspi, A. 2015, ApJ, 813, 32; DOI 10.1088/0004-637X/813/1/32
Last Modified: 2015-11-02 10:52
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Hard X-Ray Imaging of Individual Spectral Components in Solar Flares  

Amir Caspi   Submitted: 2015-09-11 18:37

We present a new analytical technique, combining Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) high-resolution imaging and spectroscopic observations, to visualize solar flare emission as a function of spectral component (e.g., isothermal temperature) rather than energy. This computationally inexpensive technique is applicable to all spatially-invariant spectral forms and is useful for visualizing spectroscopically-determined individual sources and placing them in context, e.g., comparing multiple isothermal sources with nonthermal emission locations. For example, while extreme ultraviolet images can usually be closely identified with narrow temperature ranges, due to the emission being primarily from spectral lines of specific ion species, X-ray images are dominated by continuum emission and therefore have a broad temperature response, making it difficult to identify sources of specific temperatures regardless of the energy band of the image. We combine RHESSI calibrated X-ray visibilities with spatially-integrated spectral models including multiple isothermal components to effectively isolate the individual thermal sources from the combined emission and image them separately. We apply this technique to the 2002 July 23 X4.8 event studied in prior works, and image for the first time the super-hot and cooler thermal sources independently. The super-hot source is farther from the footpoints and more elongated throughout the impulsive phase, consistent with an in situ heating mechanism for the super-hot plasma.

Authors: Amir Caspi, Albert Y. Shih, James M. McTiernan, Säm Krucker
Projects: RHESSI

Publication Status: Published -- Caspi, A., Shih, A. Y., McTiernan, J. M., & Krucker, S. 2015, ApJL, 811, L1; DOI 10.1088/2041-8205/811/1/L1
Last Modified: 2015-09-14 16:10
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New Observations of the Solar 0.5-5 keV Soft X-ray Spectrum  

Amir Caspi   Submitted: 2015-03-18 12:55

The solar corona is orders of magnitude hotter than the underlying photosphere, but how the corona attains such high temperatures is still not understood. Soft X-ray (SXR) emission provides important diagnostics for thermal processes in the high-temperature corona, and is also an important driver of ionospheric dynamics at Earth. There is a crucial observational gap between ~0.2 and ~4 keV, outside the ranges of existing spectrometers. We present observations from a new SXR spectrometer, the Amptek X123-SDD, which measured the spatially integrated solar spectral irradiance from ~0.5 to ~5 keV, with ~0.15 keV FWHM resolution, during sounding rocket flights on 2012 June 23 and 2013 October 21. These measurements show that the highly variable SXR emission is orders of magnitude greater than that during the deep minimum of 2009, even with only weak activity. The observed spectra show significant high-temperature (5-10 MK) emission and are well fit by simple power-law temperature distributions with indices of ~6, close to the predictions of nanoflare models of coronal heating. Observations during the more active 2013 flight indicate an enrichment of low first-ionization potential elements of only ~1.6, below the usually observed value of ~4, suggesting that abundance variations may be related to coronal heating processes. The XUV Photometer System Level 4 data product, a spectral irradiance model derived from integrated broadband measurements, significantly overestimates the spectra from both flights, suggesting a need for revision of its non-flare reference spectra, with important implications for studies of Earth ionospheric dynamics driven by solar SXRs.

Authors: Amir Caspi, Thomas N. Woods, Harry P. Warren
Projects: Other,SDO-EVE

Publication Status: Published -- Caspi, A., Woods, T. N., & Warren, H. P. 2015, ApJL, 802, L2; DOI 10.1088/2041-8205/802/1/L2
Last Modified: 2015-03-20 05:51
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Constraining solar flare differential emission measures with EVE and RHESSI  

Amir Caspi   Submitted: 2014-05-27 15:48

Deriving a well-constrained differential emission measure (DEM) distribution for solar flares has historically been difficult, primarily because no single instrument is sensitive to the full range of coronal temperatures observed in flares, from <2 to >50 MK. We present a new technique, combining extreme ultraviolet (EUV) spectra from the EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory with X-ray spectra from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), to derive, for the first time, a self-consistent, well-constrained DEM for jointly-observed solar flares. EVE is sensitive to ~2-25 MK thermal plasma emission, and RHESSI to >10 MK; together, the two instruments cover the full range of flare coronal plasma temperatures. We have validated the new technique on artificial test data, and apply it to two X-class flares from solar cycle 24 to determine the flare DEM and its temporal evolution; the constraints on the thermal emission derived from the EVE data also constrain the low-energy cutoff of the non-thermal electrons, a crucial parameter for flare energetics. The DEM analysis can also be used to predict the soft X-ray flux in the poorly-observed ~0.4-5 nm range, with important applications for geospace science.

Authors: Amir Caspi, James M. McTiernan, Harry P. Warren
Projects: RHESSI,SDO-EVE

Publication Status: Published -- Caspi, A., McTiernan, J. M., & Warren, H. P. 2014, ApJL, 788, L31; DOI 10.1088/2041-8205/788/2/L31
Last Modified: 2015-01-26 10:04
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Statistical Properties of Super-hot Solar Flares  

Amir Caspi   Submitted: 2013-12-02 13:09

We use RHESSI high-resolution imaging and spectroscopy observations from ~6 to 100 keV to determine the statistical relationships between measured parameters (temperature, emission measure, etc.) of hot, thermal plasma in 37 intense (GOES M- and X-class) solar flares. The RHESSI data, most sensitive to the hottest flare plasmas, reveal a strong correlation between the maximum achieved temperature and the flare GOES class, such that ?super-hot? temperatures >30 MK are achieved almost exclusively by X-class events; the observed correlation differs significantly from that of GOES-derived temperatures, and from previous studies. A nearly-ubiquitous association with high emission measures, electron densities, and instantaneous thermal energies suggests that super-hot plasmas are physically distinct from cooler, ~10?20 MK GOES plasmas, and that they require substantially greater energy input during the flare. High thermal energy densities suggest that super-hot flares require strong coronal magnetic fields, exceeding ~100 G, and that both the plasma beta and volume filling factor f cannot be much less than unity in the super-hot region.

Authors: Amir Caspi, Säm Krucker, R. P. Lin
Projects: RHESSI

Publication Status: Published -- Caspi, A., Krucker, S., & Lin, R. P. 2014, ApJ, 781, 43; DOI 10.1088/0004-637X/781/1/43
Last Modified: 2014-05-30 13:59
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Energy Release and Particle Acceleration in Flares: Summary and Future Prospects  

Amir Caspi   Submitted: 2011-10-28 12:23

RHESSI measurements relevant to the fundamental processes of energy releaseand particle acceleration in flares are summarized. RHESSI's precisemeasurements of hard X-ray continuum spectra enable model-independentdeconvolution to obtain the parent electron spectrum. Taking into account theeffects of albedo, these show that the low energy cut-off to the electronpower-law spectrum is typically below tens of keV, confirming that theaccelerated electrons contain a large fraction of the energy released inflares. RHESSI has detected a high coronal hard X-ray source that is filledwith accelerated electrons whose energy density is comparable to themagnetic-field energy density. This suggests an efficient conversion of energy,previously stored in the magnetic field, into the bulk acceleration ofelectrons. A new, collisionless (Hall) magnetic reconnection process has beenidentified through theory and simulations, and directly observed in space andin the laboratory; it should occur in the solar corona as well, with areconnection rate fast enough for the energy release in flares. Thereconnection process could result in the formation of multiple elongatedmagnetic islands, that then collapse to bulk-accelerate the electrons, rapidlyenough to produce the observed hard X-ray emissions. RHESSI's pioneering{gamma}-ray line imaging of energetic ions, revealing footpoints straddling aflare loop arcade, has provided strong evidence that ion acceleration is alsorelated to magnetic reconnection. Flare particle acceleration is shown to havea close relationship to impulsive Solar Energetic Particle (SEP) eventsobserved in the interplanetary medium, and also to both fast coronal massejections and gradual SEP events.

Authors: R.P. Lin
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Lin, R. P. 2011, Space Sci. Rev., 159, 421; DOI 10.1007/s11214-011-9801-0
Last Modified: 2011-11-17 14:21
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Deducing Electron Properties From Hard X-Ray Observations  

Amir Caspi   Submitted: 2011-10-28 12:22

X-radiation from energetic electrons is the prime diagnostic offlare-accelerated electrons. The observed X-ray flux (and polarization state)is fundamentally a convolution of the cross-section for the hard X-ray emissionprocess(es) in question with the electron distribution function, which is inturn a function of energy, direction, spatial location and time. To address theproblems of particle propagation and acceleration one needs to infer as muchinformation as possible on this electron distribution function, through adeconvolution of this fundamental relationship. This review presents recentprogress toward this goal using spectroscopic, imaging and polarizationmeasurements, primarily from the Reuven Ramaty High Energy Solar SpectroscopicImager (RHESSI). Previous conclusions regarding the energy, angular (pitchangle) and spatial distributions of energetic electrons in solar flares arecritically reviewed. We discuss the role and the observational evidence ofseveral radiation processes: free-free electron-ion, free-freeelectron-electron, free-bound electron-ion bremsstrahlung, photoelectricabsorption and Compton back-scatter (albedo), using both spectroscopic andimaging techniques. This unprecedented quality of data allows for the firsttime inference of the angular distributions of the X-ray-emitting electronsusing albedo, improved model-independent inference of electron energy spectraand emission measures of thermal plasma. Moreover, imaging spectroscopy hasrevealed hitherto unknown details of solar flare morphology and detailedspectroscopy of coronal, footpoint and extended sources in flaring regions.Additional attempts to measure hard X-ray polarization were not sufficient toput constraints on the degree of anisotropy of electrons, but point to theimportance of obtaining good quality polarization data.

Authors: E.P. Kontar, J.C. Brown, A.G. Emslie, W. Hajdas, G.D. Holman, G. J. Hurford, J. Kasparova, P. C. V. Mallik, A. M. Massone, M. L. McConnell, M. Piana, M. Prato, E. J. Schmahl, E. Suarez-Garcia
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Kontar, E. P., et al. 2011, Space Sci. Rev., 159, 301; DOI 10.1007/s11214-011-9804-x
Last Modified: 2011-11-17 14:20
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Recent Advances in Understanding Particle Acceleration Processes in Solar Flares  

Amir Caspi   Submitted: 2011-10-28 12:22

We review basic theoretical concepts in particle acceleration, withparticular emphasis on processes likely to occur in regions of magneticreconnection. Several new developments are discussed, including detailedstudies of reconnection in three-dimensional magnetic field configurations(e.g., current sheets, collapsing traps, separatrix regions) and stochasticacceleration in a turbulent environment. Fluid, test-particle, andparticle-in-cell approaches are used and results compared. While these studiesshow considerable promise in accounting for the various observationalmanifestations of solar flares, they are limited by a number of factors, mostlyrelating to available computational power. Not the least of these issues is theneed to explicitly incorporate the electrodynamic feedback of the acceleratedparticles themselves on the environment in which they are accelerated. A briefprognosis for future advancement is offered.

Authors: Valentina V. Zharkova, Karpar Arzner, Arnold O. Benz, Philippa Browning, Cyril Dauphin, A. Gordon Emslie, Lyndsay Fletcher, Eduard P. Kontar, Gottfried Mann, Marco Onofri, Vahé Petrosian, Rim Turkmani, Nicole Vilmer, Loukas Vlahos
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Zharkova, V. V., et al. 2011, Space Sci. Rev., 159, 357; DOI 10.1007/s11214-011-9803-y
Last Modified: 2011-11-17 14:20
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Abstracts by Author
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Energy Release and Particle Acceleration in Flares: Summary and Future Prospects
Deducing Electron Properties From Hard X-Ray Observations
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
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Properties of Energetic Ions in the Solar Atmosphere from γ-Ray and Neutron Observations
Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares
An Observational Overview of Solar Flares
High-Energy Aspects of Solar Flares: Overview of the Volume
Origin of the submillimeter radio emission during the time-extended phase of a solar flare
RHESSI Line and Continuum Observations of Super-hot Flare Plasma
Super-hot (T > 30 MK) Thermal Plasma in Solar Flares

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