Electron Acceleration at a Low-mach-number Perpendicular Collisionless Shock

A full particle simulation study is carried out on the electron acceleration at a collisionless, relatively low Alfven Mach number (MA = 5), perpendicular shock. Recent self-consistent hybrid shock simulations have demonstrated that the shock front of perpendicular shocks has a dynamic rippled character along the shock surface of low-Mach-number perpendicular shocks. In this paper, the effect of the rippling of perpendicular shocks on the electron acceleration is examined by means of large-scale (ion-scale) two-dimensional full particle simulations. It has been shown that a large-amplitude electric field is excited at the shock front in association with the ion-scale rippling, and that reflected ions are accelerated upstream at a localized region where the shock-normal electric field of the rippled structure is polarized upstream. The current-driven instability caused by the highly-accelerated reflected ions has a high growth rate to large-amplitude electrostatic waves. Energetic electrons are then generated by the large-amplitude electrostatic waves via electron surfing acceleration at the leading edge of the shock transition region. The present result suggests that the electron surfing acceleration is also a common feature at low-Mach-number perpendicular collisionless shocks. Subject headings: acceleration of particles — plasmas — shock waves —