Collisionless relaxation of ion distributions downstream of laminar quasi-perpendicular shocks

Directed flow of incident ions provides the free energy which is redistributed in a shock among heated ions and electrons, accelerated particles, and magnetic compression. In low Mach number laminar shock the main channel of conversion is into downstream gyrating ions. Just behind the shock transition the ion distribution is substantially nongyrotropic, which results in spatially periodic variations of the ion pressure and, consequently, in time stationary downstream oscillations of the magnetic field. In the absence of significant level of nonstationarity, gyrotropization is due to the gyrophase mixing and slow. Theoretical analysis of the phenomenon and supporting hybrid simulations are presented. It is shown that these oscillations are more likely to be observed at low Mach number low β shocks, while at higher Mach numbers or higher β they may be obscured by waves crossing the shocks.