First-order Fermi acceleration in solar flares as a mechanism for the second-step acceleration of prompt protons and relativistic electrons

Abstract

We find that for two of the hard x-ray bursts of an energetic flare on 1980 June 27, the time profile of the hard x-rays above 235 keV is delayed by 3 s with respect to the time profiles of the lower energy x-rays and that the high energy spectrum becomes flatter with time during each of these bursts. From these findings we argue that during this flare a second-step mechanism accelerated further some of the high-energy tail population of the first-step electrons. By noticing that all of the flares with second-step delays produced interplanetary energetic protons, and that gamma-ray lines wre detected from all of these flres except one that was not observed by any gamma-ray detector, we conclude that the second-step mechanism accelerates not only (mildly) relativistic electrons but also protons and heavy nuclei. Small delays of the nuclear gamma-ray time profiles with respect to the hard x-ray time profiles observed by SMM from the 1980 June 7 and 21 flares are consistent with this conclusion. After estimating the aceleration rate, we conclude that first-order Fermi acceleration operating in a closed flare loop is a very likely mechanism for the second-step acceleration.