Electron scale instabilities in the foot of a perpendicular shock

Abstract:

It is known that a variety of microscale instabilities get excited in the foot of a quasi-perpendicular shock. Two of the major instabilities which have been extensively studied for the parameters of typical heliospheric shocks are the electron cyclotron-drift instability (ECDI) and the modified two-stream instability (MTSI). They have been often discussed separately, because of the large difference in dominant wave frequency between them. Although only a few of the past studies tried to examine the nonlinear evolutions and competing processe of them, the physical parameters used in numerical simulations were not realisitic. The relative importance of the instabilities may be a function of ion-to-electron mass ratio as well as the ratio of electron plasma to cyclotron frequiencies, while these two ratios are hard to be simultaneously realistic in a full particle-in-cell simulation due to the limited computational resources. In this study we perform a number of local simulations representing a part of the foot region with systematically changing the two ratios by using two-dimensional full particle-in-cell code. The foot plasma is assumed to be consist of incoming ions, electrons, and reflected ions. Typically, the system size is smaller than ion gyro radius in X, which is parallel to the shock normal, and a few times ion inertial length in Y, which is along a perpendicular shock surface and the ambient magnetic field. The boundary conditions are periodic in both directions. We will report the results of the simulations in which the mass ratio and the frequency ratio are systematically varied with fixing the local Alfven Mach number, plasma beta, and relative density of the reflected ions.