The Formation of KappaDistribution Accelerated Electron Populations in Solar Flares 

Eduard Kontar Submitted: 20141007 02:29
Driven by recent RHESSI observations of confined looptop hard Xray 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 forwards in velocity space, so that electrons of higher energy are accelerated later; the acceleration time scales with energy according to τ_{ acc} ~ E^{3/2}. At sufficiently high energies escape from the finitelength acceleration region will eventually dominate. For such energies, the electron velocity distribution function is obtained by solving a timedependent FokkerPlanck equation in the "leakybox" 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: Bian, Nicolas H., Emslie, A. Gordon, Stackhouse, Duncan J., Kontar, Eduard P.
Projects: RHESSI

Publication Status: accepted in ApJ
Last Modified: 20141007 10:58


