Nonstationarity of a two-dimensional perpendicular shock: Competing mechanisms

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 beta(i) value, the whistler waves emission (WWE) persists for high M-A above a critical Mach number (i.e., M-A >= M-A(WWE)). The quasi-stationarity is only apparent and disappears when considering the full 3-D field profiles. Second, for lower M-A, the self-reformation is retrieved and becomes dominant as the amplitude of the whistler waves becomes negligible. Third, there exists a transition regime in M-A within which both processes compete each other. Fourth, these results are observed for a strictly perpendicular shock only as B-0 is within the simulation plane. When B-0 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.