Stochastic Acceleration of Electrons. I. Effects of Collisions in Solar Flares
Abstract
Stochastic acceleration of thermal electrons to nonrelativistic energies is studied under solar flare conditions. We show that, in turbulent regions, electronwhistler wave interactions can result in the acceleration of electrons in times comparable to or shorter than the Coulomb collision time. The kinetic equation describing the evolution of the electron energy distribution including stochastic acceleration by whistlers and energy loss via Coulomb interactions is solved for an initial thermal electron energy spectrum. In general, the shape of the resulting electron distributions are characterized by the energy E(c) where systematic energy gain by turbulence equals energy loss due to Coulomb collisions. For energies less than E(c), the spectra are steep (quasithermal) whereas above E(c), the spectra are power laws. We find that hard Xray spectra computed using the electron distributions obtained from our numerical simulations are able to explain the complex spectral shapes and variations observed in impulsive hard Xray bursts. In particular, we show that the gradual steepening observed by Lin et al. (1981) could be due to a systematic increase in the density of the plasma (due to evaporation) and the increasing importance of collisions instead of the appearance of a superhot thermal component.
 Publication:

The Astrophysical Journal
 Pub Date:
 October 1992
 DOI:
 10.1086/171860
 Bibcode:
 1992ApJ...398..350H
 Keywords:

 Electron Acceleration;
 Particle Collisions;
 Solar Flares;
 Solar Radio Bursts;
 Electron Energy;
 Energy Distribution;
 Stellar Models;
 Wave Interaction;
 Whistlers;
 X Ray Spectra;
 Astrophysics;
 ACCELERATION OF PARTICLES;
 SUN: FLARES;
 XRAYS: BURSTS