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Title: A STATISTICAL STUDY OF THE SPECTRAL HARDENING OF CONTINUUM EMISSION IN SOLAR FLARES

The observed hard X-ray and {gamma}-ray continuum in solar flares is interpreted as Bremsstrahlung emission of accelerated non-thermal electrons. It has been noted for a long time that in many flares the energy spectra show hardening at energies around or above 300 keV. In this paper, we first conduct a survey of spectral hardening events that were previously studied in the literature. We then perform a systematic examination of 185 flares from the Solar Maximum Mission. We identify 23 electron-dominated events whose energy spectra show clear double power laws. A statistical study of these events shows that the spectral index below the break ({gamma}{sub 1}) anti-correlates with the break energy ({epsilon}{sub b}). Furthermore, {gamma}{sub 1} also anti-correlates with Fr, the fraction of photons above the break compared to the total photons. A hardening spectrum, as well as the correlations between ({gamma}{sub 1}, {epsilon}{sub b}) and ({gamma}{sub 1}, Fr), provide stringent constraints on the underlying electron acceleration mechanism. Our results support a recent proposal that electrons are being accelerated diffusively at a flare termination shock with a width of the order of an ion inertial length scale.
Authors:
;  [1] ;  [2]
  1. Institute of Space Sciences and School of Space Science and Physics, Shandong University, Weihai 264209 (China)
  2. Department of Physics and CSPAR, University of Alabama in Huntsville, Huntsville, AL 35899 (United States)
Publication Date:
OSTI Identifier:
22133860
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 774; 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; BREMSSTRAHLUNG; ELECTRONS; ENERGY SPECTRA; GAMMA RADIATION; HARD X RADIATION; LIMITING VALUES; PHOTONS; SOLAR FLARES; SPECTRAL HARDENING; STATISTICS