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Scientific considerations for future spectroscopic measurements from space of activity on the Sun  

Gordon Holman   Submitted: 2016-12-23 11:11

High-resolution UV and X-ray spectroscopy are important to understanding the origin and evolution of magnetic energy release in the solar atmosphere, as well as the subsequent evolution of heated plasma and accelerated particles. Electromagnetic radiation is observed from plasma heated to temperatures ranging from about 10 kK to above 10 MK, from accelerated electrons emitting photons primarily at X-ray energies, and from ions emitting in γ rays. These observations require space-based instruments sensitive to emissions at wavelengths shorter than the near UV. This article reviews some recent observations with emphasis on solar eruptive events, the models that describe them, and the measurements they indicate are needed for substantial progress in the future. Specific examples are discussed demonstrating that imaging spectroscopy with a cadence of seconds or better is needed to follow, understand, and predict the evolution of solar activity. Critical to substantial progress is the combination of a judicious choice of UV, EUV, and soft X-ray imaging spectroscopy sensitive to the evolution of this thermal plasma combined with hard X-ray imaging spectroscopy sensitive to suprathermal electrons. The major challenge will be to conceive instruments that, within the bounds of possible technologies and funding, have the flexibility and field of view to obtain spectroscopic observations where and when events occur while providing an optimum balance of dynamic range, spectral resolution and range, and spatial resolution.

Authors: Gordon D. Holman
Projects: GOES X-rays,Hinode/EIS,IRIS,Other,RHESSI,SDO-AIA

Publication Status: Published in the Journal of Geophysical Research: Space Physics, 15 Dec 2016
Last Modified: 2016-12-28 11:41
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Direct Spatial Association of an X-Ray Flare with the Eruption of a Solar Quiescent Filament  

Gordon Holman   Submitted: 2015-04-13 12:24

Solar flares primarily occur in active regions. Hard X-ray flares have been found to occur only in active regions. They are often associated with the eruption of active region filaments and coronal mass ejections (CMEs). CMEs can also be associated with the eruption of quiescent filaments, not located in active regions. Here we report the first identification of a solar X-ray flare outside an active region observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The X-ray emission was directly associated with the eruption of a long, quiescent filament and fast CME. Images from RHESSI show this flare emission to be located along a section of the western ribbon of the expanding, post-eruption arcade. EUV images from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) show no connection between this location and nearby active regions. Therefore the flare emission is found to not be located in or associated with an active region. However, a nearby, small, magnetically strong dipolar region provides a likely explanation for the existence and location of the flare X-ray emission. This emerging dipolar region may have also triggered the filament eruption.

Authors: Gordon D. Holman & Adi Foord
Projects: GOES X-rays ,RHESSI,SDO-AIA,SDO-HMI

Publication Status: Astrophysical Journal, in press
Last Modified: 2015-04-14 07:25
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Understanding the Impact of Return-Current Losses on the X-Ray Emission from Solar Flares  

Gordon Holman   Submitted: 2011-12-05 10:23

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Gordon D. Holman
Projects: RHESSI

Publication Status: in press
Last Modified: 2011-12-05 20:23
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Subject will be restored when possible  

Gordon Holman   Submitted: 2007-10-17 15:03

We present observations of a C9.4 flare on 2002 June 2 in EUV (TRACE) and X-rays (RHESSI). The multiwavelength data reveal: (1) the involvement of a quadrupole magnetic configuration; (2) loop expansion and ribbon motion in the pre-impulsive phase; (3) gradual formation of a new compact loop with a long cusp at the top during the impulsive phase of the flare; (4) appearance of a large, twisted loop above the cusp expanding outward immediately after the hard X-ray peak; and (5) X-ray emission observed only from the new compact loop and the cusp. In particular, the gradual formation of an EUV cusp feature is very clear. The observations also reveal the timing of the cusp formation and particle acceleration: most of the impulsive hard X-rays (>25 keV) were emitted before the cusp was seen. This suggests that fast reconnection occurred during the restructuring of the magnetic configuration, resulting in more efficient particle acceleration, while the reconnection slowed after the cusp was completely formed and the magnetic geometry was stabilized. This observation is consistent with the observations obtained with Yohkoh/Soft X-ray Telescope (SXT) that soft X-ray cusp structures only appear after the major impulsive energy release in solar flares. These observations have important implications for the modeling of magnetic reconnection and particle acceleration.  2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Authors: Linhui Sui, Gordon D. Holman, & Brian R. Dennis
Projects: RHESSI,TRACE

Publication Status: Adv. Space Res. (2007), in press, doi:10.1016/j.asr.2007.03.031
Last Modified: 2007-10-18 08:18
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Subject will be restored when possible  

Gordon Holman   Submitted: 2007-10-17 12:55

The determination of the low-energy cutoff to nonthermal electron distributions is critical to the calculation of the nonthermal energy in solar flares. The most direct evidence for low-energy cutoffs is flattening of the power-law, nonthermal X-ray spectra at low energies. However, because of the plasma preheating often seen in flares, the thermal emissions at low energies may hide such spectral flattening of the nonthermal component. We select a category of flares, which we call ''early impulsive flares,'' in which the >~25 keV hard X-ray (HXR) flux increase is delayed by less than 30 s after the flux increase at lower energies. Thus, the plasma preheating in these flares is minimal, so the nonthermal spectrum can be determined to lower energies than in flares with significant preheating. Out of a sample of 33 early impulsive flares observed by the Ramaty High Energy Solar Spectroscopy Imager (RHESSI), nine showed spectral flattening toward low energies. In these events, the break energy of the double power-law fit to the HXR spectra lies in the range 10-50 keV, significantly lower than the value we have seen for other flares that do not show such early impulsive emissions. In particular, it correlates with the HXR flux. After correcting the spatially-integrated spectra for albedo from isotropically emitted X-rays and using RHESSI imaging spectroscopy to exclude the extended albedo halo, we find that albedo associated with isotropic or nearly isotropic electrons can only account for the spectral flattening in three flares near Sun center. The spectral flattening in the remaining six flares is found to be consistent with the existence of a low-energy cutoff in the electron spectrum, falling in the range 15-50 keV, which also correlates with the HXR flux.

Authors: Linhui Sui, Gordon D. Holman, and Brian R. Dennis
Projects: RHESSI

Publication Status: ApJ, in press, vol. 670 (2007 November 20)
Last Modified: 2007-10-17 13:01
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The Effects of Low- and High-Energy Cutoffs on Solar Flare Microwave and Hard X-Ray Spectra  

Gordon Holman   Submitted: 2002-11-27 13:35

Microwave and hard x-ray spectra provide crucial information about energetic electrons and their environment in solar flares. Both microwave and hard x-ray spectra are sensitive to cutoffs in the electron distribution function. The determination of the high-energy cutoff from these spectra establishes the highest electron energies produced by the acceleration mechanism, while determination of the low-energy cutoff is crucial to establishing the total energy in accelerated electrons. I present computations of the effects of both high- and low-energy cutoffs on microwave and hard x-ray spectra. The optically thick portion of a microwave spectrum is enhanced and smoothed by a low-energy cutoff, while a hard x-ray spectrum is flattened below the cutoff energy. A high-energy cutoff steepens the microwave spectrum and increases the wavelength at which the spectrum peaks, while the hard x-ray spectrum begins to steepen at photon energies an order of magnitude or more below the electron cutoff energy. I discuss how flare microwave and hard x-ray spectra can be analyzed together to determine these electron cutoff energies.

Authors: Gordon D. Holman
Projects:

Publication Status: Ap. J., in press, 2003
Last Modified: 2002-11-27 13:35
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Modeling Images and Spectra of a Solar Flare Observed by RHESSI on 20 February 2002  

Gordon Holman   Submitted: 2002-09-19 15:15

We have analyzed a C7.5 limb flare observed by RHESSI on 20 February 2002. The RHESSI images appear to show two footpoints and a looptop source. Our goal was to determine if the data are consistent with a simple steady-state model in which high-energy electrons are continuously injected at the top of a semicircular flare loop. A comparison of the RHESSI images with simulated images from the model has made it possible for us to identify spurious sources and fluxes in the RHESSI images. We find the RHESSI results are in many aspects consistent with the model if a thermal source is included between the loop footpoints, but there is a problem with the spectral index of the looptop source. The thermal source between the footpoints is likely to be a low-lying loop interacting with the northern footpoint of a higher loop containing the looptop source.

Authors: L. Sui, G. D. Holman, B. R. Dennis, S. Krucker, R. A. Schwartz, and K. Tolbert
Projects:

Publication Status: Solar Physics (in press)
Last Modified: 2002-09-19 15:58
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Abstracts by Author
Scientific considerations for future spectroscopic measurements from space of activity on the Sun
Direct Spatial Association of an X-Ray Flare with the Eruption of a Solar Quiescent Filament
Understanding the Impact of Return-Current Losses on the X-Ray Emission from Solar Flares
Subject will be restored when possible
Subject will be restored when possible
The Effects of Low- and High-Energy Cutoffs on Solar Flare Microwave and Hard X-Ray Spectra
Modeling Images and Spectra of a Solar Flare Observed by RHESSI on 20 February 2002

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