Determination of Stochastic Acceleration Model Characteristics in Solar Flares 

Qingrong Chen Submitted: 20130709 15:23
Following our recent paper (Petrosian & Chen 2010), we have developed an inversion method to determine the basic characteristics for the model of stochastic acceleration
of particles by plasma waves or turbulence directly and nonparametrically from observations in the framework of
the leaky box version of the FokkerPlanck kinetic equation.
These characteristics are determined by waveparticle interactions as described by the momentum and pitch angle diffusion coefficients. Our method relates these coefficients directly to observations. In particular,
we show that by inverting the FokkerPlanck equation to its integral form, one can derive the energy diffusion coefficient and direct acceleration rate by turbulence
in terms of the accelerated and escaping particle spectra.
We apply the analytic formulas to the suprathermal electrons
in solar flares, which produce hard Xray emission at the coronal loop top (LT) region and two thick target footpoint regions of the flare loop. We utilize the regularized electron flux spectral images recently developed for
the Reuven Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}). From the spatially resolved electron flux spectra, we determine the electron escape time, which is related to the pitch angle scattering rate, and the energy diffusion coefficient at the LT acceleration region.
Results obtained from two relatively intense {it RHESSI} events indicate that the escape time increases with energy and the energy diffusion (or direct acceleration) time
and scattering time have dramatically different energy dependences. Such behaviors may be difficult to explain by
existing waveparticle interaction models, and may indicate that a different acceleration mechanism is at work or imply a breakdown of the basic interpretation of the escape of electrons being a random walk process. The discrepant energy dependences can be alleviated somewhat by a turbulence spectrum that is much steeper than the Kolmogorovtype spectrum. On the other hand, a more likely explanation could be that the escape of electrons out of the LT acceleration region is governed by converging field lines in a magnetic mirror geometry, in which the escape time is proportional to scattering time. The results demonstrate the critical importance of combined modeling of
electron acceleration by plasma wave turbulence and the large scale magnetic field variations in a reconnection environment.
Authors: Qingrong Chen and Vahé Petrosian
Projects: RHESSI

Publication Status: submitted to ApJ
Last Modified: 20130710 13:40


