Pre-acceleration in collapsing magnetic neutral sheets and composition anomalies among solar flare particles

In flares occurring in magnetic neutral sheets, the collapsing of the magnetic fields toward the neutral sheet results in their acting as mirrors in a first-order Fermi acceleration process and constitutes an intrinsic source of particle pre-acceleration. Although modest, the acceleration injects particles into the main flare acceleration with a composition which is different from that of the ambient corona. Upon calculating the fractions of 18 elements which are accelerated in the collapsing mirrors, covering the series from C to N, and then normalizing to O, peak abundances are found at Si and Fe, with local minima at N and in the S-Ca region. There is a qualitative similarity between these relative abundances and the relative enhancement factors of the elements among solar flare particles.