ION DYNAMICS IN HIGH-MACH-NUMBER QUASI-PARALLEL SHOCKS

Ion heating in high-Mach-number (MA greater-than-or-equal-to 3) quasi-parallel shocks is accomplished by ion reflections at the shock ramp and nonadiabatic pitch angle scattering through nonlinear waves in the shock transition region. Two types of ion reflection events are observed in high-Mach-number shock simulations: (1) the reflecting-reentering event and (2) the backstreaming-gyrating event. The behavior of ion reflection depends on the orientation of the ramp field relative to the upstream field. A gyroreflection model is proposed to provide a physical explanation for the two types of ion reflection events observed in the high-Mach-number shocks. The rotation of the ramp field with time leads to alternative occurrence of the two types of ion reflection events, which in turn leads to shock front reformation in the high-Mach-number quasi-parallel shock. The time scale for the shock front reformation is estimated to be about 1 to 2 upstream ion gyroperiods, which is about 10 ion gyroperiods for ions experiencing overshot magnetic field in the shock ramp.