Global energetics of solar flares. III. Nonthermal energies

Aschwanden, M. J., Holman, G., O’Flannagain, A., Caspi, A., McTiernan, J. M. and Kontar, E. P. (2016) Global energetics of solar flares. III. Nonthermal energies. Astrophysical Journal, 832(1), 27. (doi:10.3847/0004-637X/832/1/27)

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Abstract

This study entails the third part of a global flare energetics project, in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data of 191 M and X-class flare events from the first 3.5 years of the Solar Dynamics Observatory mission are analyzed. We fit a thermal and a nonthermal component to RHESSI spectra, yielding the temperature of the differential emission measure (DEM) tail, the nonthermal power-law slope and flux, and the thermal/nonthermal cross-over energy eco. From these parameters, we calculate the total nonthermal energy Ent in electrons with two different methods: (1) using the observed cross-over energy eco as low-energy cutoff, and (2) using the low-energy cutoff ewt predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of Te = 8.6 MK in active regions, we find low-energy cutoff energies of ewt =  6.2 1.6 keV for the warm-target model, which is significantly lower than the cross-over energies eco =  21 6 keV. Comparing with the statistics of magnetically dissipated energies Emag and thermal energies Eth from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: Ent mag = 0.41 E , Eth mag = 0.08 E , and Eth n = 0.15 E t. The total dissipated magnetic energy exceeds the thermal energy in 95% and the nonthermal energy in 71% of the flare events, which confirms that magnetic reconnection processes are sufficient to explain flare energies. The nonthermal energy exceeds the thermal energy in 85% of the events, which largely confirms the warm thick-target model.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Kontar, Dr Eduard
Authors: Aschwanden, M. J., Holman, G., O’Flannagain, A., Caspi, A., McTiernan, J. M., and Kontar, E. P.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astrophysical Journal
Publisher:IOP Publishing
ISSN:0004-637X
ISSN (Online):1538-4357
Published Online:14 November 2016
Copyright Holders:Copyright © 2016 The American Astronomical Society
First Published:First published in Astrophysical Journal 832(1): 27
Publisher Policy:Reproduced in accordance with the publisher copyright policy
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