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Title: Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles

Abstract

Flares close to the solar limb, where the footpoints are occulted, can reveal the spectrum and structure of the coronal looptop source in X-rays. We aim at studying the properties of the corresponding energetic electrons near their acceleration site, without footpoint contamination. To this end, a statistical study of partially occulted flares observed with Reuven Ramaty High-Energy Solar Spectroscopic Imager is presented here, covering a large part of solar cycles 23 and 24. We perform detailed spectra, imaging, and light curve analyses for 116 flares and include contextual observations from SDO and STEREO when available, providing further insights into flare emission that were previously not accessible. We find that most spectra are fitted well with a thermal component plus a broken power-law, non-thermal component. A thin-target kappa distribution model gives satisfactory fits after the addition of a thermal component. X-ray imaging reveals small spatial separation between the thermal and non-thermal components, except for a few flares with a richer coronal source structure. A comprehensive light curve analysis shows a very good correlation between the derivative of the soft X-ray flux (from GOES ) and the hard X-rays for a substantial number of flares, indicative of the Neupert effect. The resultsmore » confirm that non-thermal particles are accelerated in the corona and estimated timescales support the validity of a thin-target scenario with similar magnitudes of thermal and non-thermal energy fluxes.« less

Authors:
; ;  [1]; ; ;  [2];  [3];  [4]
  1. Department of Physics and KIPAC, Stanford University, Stanford, CA 94305 (United States)
  2. Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450 (United States)
  3. Bay Area Environmental Research Institute, 625 2nd Street, Suite 209, Petaluma, CA 94952 (United States)
  4. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 (United States)
Publication Date:
OSTI Identifier:
22663855
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; CORRELATIONS; DISTRIBUTION; EMISSION; GAMMA RADIATION; HARD X RADIATION; LIMBS; PARTICLES; SOFT X RADIATION; SOLAR CYCLE; SOLAR FLARES; SPECTRA; SUN; TAIL ELECTRONS; ULTRAVIOLET RADIATION; VISIBLE RADIATION

Citation Formats

Effenberger, Frederic, Costa, Fatima Rubio da, Petrosian, Vahé, Oka, Mitsuo, Saint-Hilaire, Pascal, Krucker, Säm, Liu, Wei, and Glesener, Lindsay, E-mail: feffen@stanford.edu, E-mail: frubio@stanford.edu. Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/2/124.
Effenberger, Frederic, Costa, Fatima Rubio da, Petrosian, Vahé, Oka, Mitsuo, Saint-Hilaire, Pascal, Krucker, Säm, Liu, Wei, & Glesener, Lindsay, E-mail: feffen@stanford.edu, E-mail: frubio@stanford.edu. Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles. United States. doi:10.3847/1538-4357/835/2/124.
Effenberger, Frederic, Costa, Fatima Rubio da, Petrosian, Vahé, Oka, Mitsuo, Saint-Hilaire, Pascal, Krucker, Säm, Liu, Wei, and Glesener, Lindsay, E-mail: feffen@stanford.edu, E-mail: frubio@stanford.edu. Wed . "Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles". United States. doi:10.3847/1538-4357/835/2/124.
@article{osti_22663855,
title = {Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles},
author = {Effenberger, Frederic and Costa, Fatima Rubio da and Petrosian, Vahé and Oka, Mitsuo and Saint-Hilaire, Pascal and Krucker, Säm and Liu, Wei and Glesener, Lindsay, E-mail: feffen@stanford.edu, E-mail: frubio@stanford.edu},
abstractNote = {Flares close to the solar limb, where the footpoints are occulted, can reveal the spectrum and structure of the coronal looptop source in X-rays. We aim at studying the properties of the corresponding energetic electrons near their acceleration site, without footpoint contamination. To this end, a statistical study of partially occulted flares observed with Reuven Ramaty High-Energy Solar Spectroscopic Imager is presented here, covering a large part of solar cycles 23 and 24. We perform detailed spectra, imaging, and light curve analyses for 116 flares and include contextual observations from SDO and STEREO when available, providing further insights into flare emission that were previously not accessible. We find that most spectra are fitted well with a thermal component plus a broken power-law, non-thermal component. A thin-target kappa distribution model gives satisfactory fits after the addition of a thermal component. X-ray imaging reveals small spatial separation between the thermal and non-thermal components, except for a few flares with a richer coronal source structure. A comprehensive light curve analysis shows a very good correlation between the derivative of the soft X-ray flux (from GOES ) and the hard X-rays for a substantial number of flares, indicative of the Neupert effect. The results confirm that non-thermal particles are accelerated in the corona and estimated timescales support the validity of a thin-target scenario with similar magnitudes of thermal and non-thermal energy fluxes.},
doi = {10.3847/1538-4357/835/2/124},
journal = {Astrophysical Journal},
number = 2,
volume = 835,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2017},
month = {Wed Feb 01 00:00:00 EST 2017}
}
  • The model of stochastic acceleration of particles by turbulence has been successful in explaining many observed features of solar flares. Here, we demonstrate a new method to obtain the accelerated electron spectrum and important acceleration model parameters from the high-resolution hard X-ray (HXR) observations provided by RHESSI. In our model, electrons accelerated at or very near the loop top (LT) produce thin target bremsstrahlung emission there and then escape downward producing thick target emission at the loop footpoints (FPs). Based on the electron flux spectral images obtained by the regularized spectral inversion of the RHESSI count visibilities, we derive severalmore » important parameters for the acceleration model. We apply this procedure to the 2003 November 3 solar flare, which shows an LT source up to 100-150 keV in HXR with a relatively flat spectrum in addition to two FP sources. The results imply the presence of strong scattering and a high density of turbulence energy with a steep spectrum in the acceleration region.« less
  • Two solar flares occurring on 1978 December 3 and 4 were observed in X-rays > or approx. =100 keV by identical instruments on widely separated spacecraft operating in near-Earth orbit ( Prognoz 7) and interplanetary space (Venera 11 and 12). The data show clear evidence for fine time structure on scales down to 55 ms for the e-folding rise and fall times. Such data may be used to localize the emission region by the method of arrival time analysis.
  • We have studied the temporal and spatial structures of UV and hard X-ray bursts in a disk ad a limb flare observed with instruments on the Solar Maximum Mission. The UV observations were spatially resolved with a resolution of 3''--4'' and were done in the Si IV, O IV lines and in the UV continuum region. The results are: (1) There was considerable preflare activity with UV transient brightenings occurring in many small point-like kernels. The flare later occurred in these active kernels; some of the active kernels, however, did not flare. (2) The impulsie UV and hard X-ray burstsmore » were temporally correlated. Indivudual peaks in the hard X-ray bursts can be identified with individual peaks in the UV bursts of individual flaring kernels. In between the individual maxima, however, the UV emission remained intense. (3) There were impulsie brightenings in the UV continuum during the time of the hard X-ray bursts. (4) For the disk flare, the many flaring kernels were confined in a small area while for the limb flare, the impulsive bursts occurred in two points separated by a large distance of 50'' on the limb. (5)= The UV bursts started to rise earlier than the correspondig hard X-ray bursts. (6) There is some indication that the UV intensity of the bright points actually dropped just before it began to flare and to rise sharply. These results indicate that the emission source of the hard X-ray burst was located in the lower atmosphere where the density is high, and that there was local heating.« less
  • Observations and theoretical considerations have led to a model for impulsive phase flare emission involving the heating and acceleration of thermal electrons in the coronal part of a magnetic loop. The bulk of the heated gas is confined between conduction fronts, but particles with velocities a few times greater than the thermal velocity can escape into the lower part of the loop. We show that, when the electron gyrofrequency exceeds the plasma frequency, the escaping electrons are unstable to the generation of electrostatic plasma waves which scatter the particles in pitch angle to a nearly isotropic distribution. We show thatmore » this scattering can (i) enhance the microwave emission from the upper part of the loop and (ii), due to the Landau damping of both low and high phase velocity waves, can lead to one or two breaks in the impulsive phase hard X-ray spectrum.« less