ELECTRON ENERGY PARTITION IN THE ABOVE-THE-LOOPTOP SOLAR HARD X-RAY SOURCES

Solar flares produce non-thermal electrons with energies up to tens of MeVs. To understand the origin of energetic electrons, coronal hard X-ray (HXR) sources, in particular above-the-looptop sources, have been studied extensively. However, it still remains unclear how energies are partitioned betwe...

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Bibliographic Details
Published in:The Astrophysical journal 2015-02, Vol.799 (2)
Main Authors: Oka, Mitsuo, Krucker, Säm, Hudson, Hugh S., Saint-Hilaire, Pascal
Format: Article
Language:English
Subjects:
ACCELERATION
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
COMPARATIVE EVALUATIONS
EXTREME ULTRAVIOLET RADIATION
GAMMA RADIATION
HARD X RADIATION
KEV RANGE
MAGNETIC RECONNECTION
SOLAR ELECTRONS
SOLAR FLARES
SUN
TAIL ELECTRONS
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Summary:Solar flares produce non-thermal electrons with energies up to tens of MeVs. To understand the origin of energetic electrons, coronal hard X-ray (HXR) sources, in particular above-the-looptop sources, have been studied extensively. However, it still remains unclear how energies are partitioned between thermal and non-thermal electrons within the above-the-looptop source. Here we show that the kappa distribution, when compared to conventional spectral models, can better characterize the above-the-looptop HXRs (≳15 keV) observed in four different cases. The widely used conventional model (i.e., the combined thermal plus power-law distribution) can also fit the data, but it returns unreasonable parameter values due to a non-physical sharp lower-energy cutoff E{sub c}. In two cases, extreme-ultraviolet data were available from SDO/AIA and the kappa distribution was still consistent with the analysis of differential emission measure. Based on the kappa distribution model, we found that the 2012 July 19 flare showed the largest non-thermal fraction of electron energies about 50%, suggesting equipartition of energies. Considering the results of particle-in-cell simulations, as well as density estimates of the four cases studied, we propose a scenario in which electron acceleration is achieved primarily by collisionless magnetic reconnection, but the electron energy partition in the above-the-looptop source depends on the source density. In low-density above-the-looptop regions (few times 10{sup 9} cm{sup –3}), the enhanced non-thermal tail can remain and a prominent HXR source is created, whereas in higher-densities (>10{sup 10} cm{sup –3}), the non-thermal tail is suppressed or thermalized by Coulomb collisions.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/799/2/129