Sources of two-dimensional supercritical perpendicular shock nonstationarity: nonlinear whistler emission versus shock front self-reformation
Résumé
Two-dimensional (2-D) full particle electromagnetic simulations are used for analysing in detail different nonstationary behaviors of perpendicular supercritical shocks (Lembège et al., 2009). A recent study (Hellinger et al., 2007) has previously evidenced that the shock front is dominated by the emission of coherent large amplitude whistler waves for some plasma conditions and shock regimes. These whistler waves are emitted in two-dimensional perpendicular shocks and inhibit the self-reformation driven by the accumulation of reflected ions: then, the shock front appears almost "quasistationary", a result which could seem in apparent contradiction with previous results. The present study allows to clarify the situation by bringing new complementary results:(i) there exists a transition regime around a critical Mach number threshold Mwwe, within which both self-reformation and whistler waves emission can co-exist. (ii) Below (above) this threshold regime, the self-reformation (whistler waves emission) is fully retrieved and becomes dominant. (iii) As MA is larger than Mwwe, this shock front looks "quasi-stationary" in 1-D y-averaged fields profiles, but in fact is nonstationary in full 2-D profiles. Moreover, this nonstationarity is characterized by a quasi-periodic reinforcement of nonlinear waves emission from the ramp. This effect results from the fact that the emission of nonlinear whistler waves varies in time according to the local need for balancing the nonlinear effects at the shock ramp (steepening). (iv) These results are observed for a strictly perpendicular shock, as B0 is within the simulation plane; in contrast, as B0 is perpendicular to the simulation plane, no whistler waves emission is evidenced even for large Mach number; only self-reformation is observed. The coexistence of both processes within the transition regime is characterized by a rapid occurrence of the self-reformation at early time of a run, while nonlinear waves are dominating the whole shock dynamics at later times of the same run. A parametric study shows that effectively the growth of these nonlinear waves is strongly dependant on the time, and increases drastically as the Mach number is above the threshold Mwwe. Present results, even if unexpected, are shown to be in a good agreement with previous 2D PIC and 2D hybrid simulations these are compared with. References : HELLINGER P., P. TRAVICEK, B. LEMBEGE and P. SAVOINI . Emission of nonlinear whistler waves at the front of perpendicular supercritical shocks: Hybrid versus full particle simulations J. Geophys. Res. , VOL. 34, L14109, doi:10.1029/2007GL030239, 2007 LEMBEGE B. , P. SAVOINI, P. HELLINGER and P.M. TRAVNICEK Nonstationarity of a two-dimensional perpendicular shock: competing mechanims J. Geophys. Res., 114, A03217, doi:10.1029/2008JA013618, 2009