Test particle acceleration in 2D current sheets undergoing dynamical phases of reconnection

Abstract

Calculations of collision-free test particle orbits in a dynamic, filamentary current sheet are presented. The electromagnetic fields are obtained from a two-dimensional MHD simulation; thus this study is restricted to energization in perpendicular electric fields. Starting from a multiple X-line initial perturbation, the current sheet undergoes complex dynamics, comprising a tearing-like phase, multiple island coalescence and plasmoid formation, and finally the acceleration and ejection of the plasmoid. The dynamic, multiple X-line configuration possesses two important properties supporting strong particle acceleration: efficient particle confinement which permits multiple or long-lasting acceleration phases, and appropriate asymmetries of the average electric field about the O-lines, which permits acceleration of the majority of test particles. Dynamic current sheets may thus be responsible for the efficient bulk acceleration of particles observed in the impulsive phase of solar flares. For parameters of the lower solar corona, sub-second acceleration times and maximum energies in the 10 7–10 8 eV range are obtained.