Scientific considerations for future spectroscopic measurements from space of activity on the Sun |
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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
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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 |
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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
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Publication Status: Astrophysical Journal, in press
Last Modified: 2015-04-14 07:25
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Subject will be restored when possible |
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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
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Publication Status: ApJ, in press, vol. 670 (2007 November 20)
Last Modified: 2007-10-17 13:01
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