High Mach Number Nonstationary Shocks: Heating and Acceleration of Electrons

Collisionless quasiperpendicular shock waves with Mach number larger than some critical value, M>M _{cr}^*, undergo a quasiperiodic reformation process that consists in overturning of the shock front and formation of the new one from the growing precursor whistler wave train upstream of the previous shock front. The overturning process results in the growth of magnetic field gradients up to very large values and in the formation of the region with extremely large amplitudes of the electric field. The electric field in the vicinity of the overturning point in MHD approximation grows explosively up to infinity. The growth can be arrested either by the kinetic effects, or by dispersion and/or dissipation. However, within a quite narrow region in the shock front the electric field can become large enough to ensure about 1/3 of the total potential jump across the shock front. This electric field increase can provide additional mechanisms of the energy dissipation and particle acceleration. Two possible effects are shown to play an important role: the growth of the efficiency of the surfing acceleration mechanism and direct acceleration due to the explosive growth of the electric field.