The shock reprocessing model of electron acceleration in impulsive solar flares

Abstract

We propose a new two-stage model for acceleration of electrons in solar flares. In the first stage, electrons are accelerated stochastically in a post-reconnection turbulent downflow. The second stage is the reprocessing of a subset of these electrons as they pass through a weakly compressive fast shock above the apex of the closed flare loop on their way to the chromosphere. We call this the "shock reprocessing" model. The model reproduces the energy dependent arrival time delays observed for both the pulsed and smooth components of impulsive solar flare x-rays with physically reasonable parameters for the downflow region. The model also predicts an emission site above the loop-top, as seen in the Masuda flare. The loop-top source distinguishes the shock reprocessing model from previous models. The model makes testable predictions for the energy dependence of footpoint pulse strengths and the location and spectrum of the loop-top emission, and can account for the observed soft-hard-soft trend in the spectral evolution of footpoint emission. Our model highlights the concept that reconnection is an acceleration environment rather than a single process. Which combination of processes operate may depend on the initial conditions that determine, for example, whether the reconnection downflow is turbulent. The shock reprocessing model comprises one such combination.