Nonstationarity of a two‐dimensional perpendicular shock: Competing mechanisms

Abstract

Two‐dimensional particle‐in‐cell (PIC) simulations are used for analyzing in detail different nonstationary behaviors of a perpendicular supercritical shock. A recent study by Hellinger et al. (2007) has shown that the front of a supercritical shock can be dominated by the emission of large‐amplitude whistler waves. These waves inhibit the self‐reformation driven by the reflected ions; then, the shock front appears almost “quasi‐stationary.” The present study stresses new complementary results. First, for a fixed βi value, the whistler waves emission (WWE) persists for high MA above a critical Mach number (i.e., MA ≥ MAWWE). The quasi‐stationarity is only apparent and disappears when considering the full 3‐D field profiles. Second, for lower MA, the self‐reformation is retrieved and becomes dominant as the amplitude of the whistler waves becomes negligible. Third, there exists a transition regime in MA within which both processes compete each other. Fourth, these results are observed for a strictly perpendicular shock only as B0 is within the simulation plane. When B0 is out of the simulation plane, no whistler waves emission is evidenced and only self‐reformation is recovered. Fifth, the occurrence and disappearance of the nonlinear whistler waves are well recovered in both 2‐D PIC and 2‐D hybrid simulations. The impacts on the results of the mass ratio (2‐D PIC simulations), of the resistivity and spatial resolution (2‐D hybrid simulations), and of the size of the simulation box along the shock front are analyzed in detail.