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Title: The formation of kappa-distribution accelerated electron populations in solar flares

Driven by recent RHESSI observations of confined loop-top hard X-ray sources in solar flares, we consider stochastic acceleration of electrons in the presence of Coulomb collisions. If electron escape from the acceleration region can be neglected, the electron distribution function is determined by a balance between diffusive acceleration and collisions. Such a scenario admits a stationary solution for the electron distribution function that takes the form of a kappa distribution. We show that the evolution toward this kappa distribution involves a 'wave front' propagating forward in velocity space, so that electrons of higher energy are accelerated later; the acceleration timescales with energy according to τ{sub acc} ∼ E {sup 3/2}. At sufficiently high energies escape from the finite-length acceleration region will eventually dominate. For such energies, the electron velocity distribution function is obtained by solving a time-dependent Fokker-Planck equation in the 'leaky-box' approximation. Solutions are obtained in the limit of a small escape rate from an acceleration region that can effectively be considered a thick target.
Authors:
; ;  [1] ;  [2]
  1. School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ (United Kingdom)
  2. Department of Physics and Astronomy, Western Kentucky University, Bowling Green, KY 42101 (United States)
Publication Date:
OSTI Identifier:
22370125
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 796; 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; APPROXIMATIONS; COSMIC X-RAY SOURCES; DISTRIBUTION; DISTRIBUTION FUNCTIONS; ELECTRONS; FOKKER-PLANCK EQUATION; GAMMA RADIATION; HARD X RADIATION; SOLAR FLARES; SPACE; STAR EVOLUTION; STOCHASTIC PROCESSES; SUN; TIME DEPENDENCE; VELOCITY