Effects of overshoots on electron distributions upstream and downstream of quasi-perpendicular collisionless shocks

摘要

We use test particle simulations to investigate the effect of overshoots on electron distributions upstream and downstream of quasi-perpendicular collisionless shocks as functions of angle bn between upstream magnetic field and shock normal. The main results are: (1) The development of overshoots leads to obvious loss cone structures in upstream electron distributions, increases the reflection of upstream electrons, and reduces the leakage of downstream electrons. An electron beam associated with the escape cutoff effect exists in upstream reduced electron distribution functions for large bn shocks. (2) Overshoots significantly affect the downstream electron distribution, too, preventing the upstream and downstream electrons with high perpendicular speeds from crossing the shock and leading to a loss cone and beam features rather than arc features in downstream distributions. (3) The shock-reflected electrons conserve their magnetic moments on average. Demagnetized electrons are mainly those transmitted, and most are overadiabatic electrons. The assumption of conservation of magnetic moment is an increasingly good approximation as bn increases. (4) The major temporal changes in ramp overshoot structure expected from shock nonstationarity should lead to significant changes in electron distributions and associated waves upstream and downstream.