Effects of Various Ion Distributions on Relaxation of the Transmitted Cold Ion Beam Downstream of Shocks

Heating and thermalization of the upstream supermagnetosonic plasma flow at collisionless supercritical shocks has significance for stability of the shock layer and particle energization both upstream and downstream of the shock. In the downstream region, gyrophase mixing of nongyrotropic ions determine the thermalization rate of the ion distributions. In this study, we revisit this problem by reassessing the role of different ion populations (i.e., directly transmitted and reflected transmitted) in relaxation of the transmitted ion beam. We do so by analyzing high resolution, multipoint measurements of particles and fields by the Magnetospheric Multiscale (MMS) spacecraft. We consider features of dominant wave modes downstream of the shock in relation to isotropization of ions, and we estimate the relative contributions of various ion populations to the perpendicular plasma pressure oscillations that lead to anti-correlated variations in the magnetic field pressure. The results will improve our understanding of energy budget and energy transfer processes downstream of the supercritical shocks.