Shock front instability associated with reflected ions at the perpendicular shock

Abstract

Two-dimensional hybrid simulations of perpendicular, supercritical collisionless shocks are carried out in a geometry with the magnetic field perpendicular to the simulation plane so that parallel propagating fluctuations, such as Alfvén ion cyclotron waves, are suppressed. In terms of average profile and large downstream ion temperature anisotropy, the results resemble those from earlier one-dimensional hybrid simulations, and differ markedly from the results of two-dimensional simulations in which field-parallel propagating fluctuations are included. In addition, we find an instability at the shock front, in which a pattern of magnetic field and density enhancements propagates along the shock surface in the direction of gyration and at the average speed of the ions reflected at the shock. The instability mechanism depends on a spatio-temporal modulation of the fraction of reflected ions over the shock surface. The instability has a threshold that depends on the Mach number and the upstream ion plasma beta, being stabilized by an increased beta and decreased Mach number. In a realistic three-dimensional planar shock, this instability will be only one of several mechanisms contributing to shock front nonstationarity. However, at a three-dimensional curved shock, there is a region where the instability mechanism described may dominate.