The role of diffusion in the transport of energetic electrons during solar flares

Bian, N. H., Emslie, A. G. and Kontar, E. P. (2017) The role of diffusion in the transport of energetic electrons during solar flares. Astrophysical Journal, 835(2), 262. (doi:10.3847/1538-4357/835/2/262)

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Abstract

The transport of the energy contained in suprathermal electrons in solar flares plays a key role in our understanding of many aspects of flare physics, from the spatial distributions of hard X-ray emission and energy deposition in the ambient atmosphere to global energetics. Historically the transport of these particles has been largely treated through a deterministic approach, in which first-order secular energy loss to electrons in the ambient target is treated as the dominant effect, with second-order diffusive terms (in both energy and angle) generally being either treated as a small correction or even neglected. Here, we critically analyze this approach, and we show that spatial diffusion through pitch-angle scattering necessarily plays a very significant role in the transport of electrons. We further show that a satisfactory treatment of the diffusion process requires consideration of non-local effects, so that the electron flux depends not just on the local gradient of the electron distribution function but on the value of this gradient within an extended region encompassing a significant fraction of a mean free path. Our analysis applies generally to pitch-angle scattering by a variety of mechanisms, from Coulomb collisions to turbulent scattering. We further show that the spatial transport of electrons along the magnetic field of a flaring loop can be modeled rather effectively as a Continuous Time Random Walk with velocity-dependent probability distribution functions of jump sizes and occurrences, both of which can be expressed in terms of the scattering mean free path.

Item Type:Articles
Additional Information:A.G.E. was supported by grant NNX10AT78G from NASA’s Goddard Space Flight Center. N.H.B. was supported by a STFC grant.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Kontar, Dr Eduard and Bian, Dr Nicolas Horace
Authors: Bian, N. H., Emslie, A. G., 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:01 February 2017
Copyright Holders:Copyright © 2017 The American Astronomical Society
First Published:First published in Astrophysical Journal 835(2): 262
Publisher Policy:Reproduced in accordance with the publisher copyright policy
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