Nonrelativistic collisionless shocks in weakly magnetized electron--ion plasmas: two-dimensional particle-in-cell simulation of perpendicular shock

摘要

A two-dimensional particle-in-cell simulation is performed to investigateweakly magnetized perpendicular shocks with a magnetization parameter of 6 x10^-5, which is equivalent to a high Alfv\'en Mach number M_A of ~130. It isshown that current filaments form in the foot region of the shock due to theion-beam--Weibel instability (or the ion filamentation instability) and thatthey generate a strong magnetic field there. In the downstream region, thesecurrent filaments also generate a tangled magnetic field that is typically 15times stronger than the upstream magnetic field. The thermal energies ofelectrons and ions in the downstream region are not in equipartition and theirtemperature ratio is T_e / T_i ~ 0.3 - 0.4. Efficient electron acceleration wasnot observed in our simulation, although a fraction of the ions are acceleratedslightly on reflection at the shock. The simulation results agree very wellwith the Rankine-Hugoniot relations. It is also shown that electrons and ionsare heated in the foot region by the Buneman instability (for electrons) andthe ion-acoustic instability (for both electrons and ions). However, the growthrate of the Buneman instability is significantly reduced due to the relativelyhigh temperature of the reflected ions. For the same reason, ion-ion streaminginstability does not grow in the foot region.