DETERMINATION OF STOCHASTIC ACCELERATION MODEL CHARACTERISTICS IN SOLAR FLARES

Abstract

Following our recent paper (Petrosian & Chen 2010), we have developed an\ninversion method to determine the basic characteristics of the particle\nacceleration mechanism directly and non-parametrically from observations under\nthe leaky box framework. In the above paper, we demonstrated this method for\nobtaining the energy dependence of the escape time. Here, by converting the\nFokker-Planck equation to its integral form, we derive the energy dependences\nof the energy diffusion coefficient and direct acceleration rate for stochastic\nacceleration in terms of the accelerated and escaping particle spectra.\nCombining the regularized inversion method of Piana et al. 2007 and our\nprocedure, we relate the acceleration characteristics in solar flares directly\nto the count visibility data from RHESSI. We determine the timescales for\nelectron escape, pitch angle scattering, energy diffusion, and direct\nacceleration at the loop top acceleration region for two intense solar flares\nbased on the regularized electron flux spectral images. The X3.9 class event\nshows dramatically different energy dependences for the acceleration and\nscattering timescales, while the M2.1 class event shows a milder difference.\nThe M2.1 class event could be consistent with the stochastic acceleration model\nwith a very steep turbulence spectrum. A likely explanation of the X3.9 class\nevent could be that the escape of electrons from the acceleration region is not\ngoverned by a random walk process, but instead is affected by magnetic\nmirroring, in which the scattering time is proportional to the escape time and\nhas an energy dependence similar to the energy diffusion time.\n