Particle acceleration during solar flares is a highly efficient process, in terms of both the energy (tens of percent of the released magnetic energy can be transferred to the energetic particles) and the particle number (nearly all electrons in the flaring region are accelerated). So-called stochastic acceleration (e.g., Petrosian 2012 for a review) is one of the most popular mechanisms capable of providing these characteristics. In this scenario, the magnetic reconnection produces turbulent outflow that excites an MHD turbulence which then accelerates the particles. The stochastic acceleration mechanism has been studied extensively in many theoretical papers, but until now it has lacked a direct observational confirmation.
Figure 1. Left: morphology of the flare; background: SDO/AIA 193 Å image, red and blue: 50% RHESSI X-ray contours at 6-15 and 25-50 keV, white: 30% and 75% Hinode/EIS 255 Å contours, green: 30% and 75% Nobeyama Radioheliograph 34 GHz contours. Right top: RHESSI X-ray and Nobeyama Radiopolarimeters light curves. Right bottom: temporal evolution of the nonthermal broadening velocity (from Hinode/EIS) averaged over the coronal X-ray source area.
We investigated a moderately large (X1.2) and relatively short flare that occurred on May 15, 2013 near the solar limb. This flare was observed in ...