Density jump as a function of magnetic field strength for perpendicular collisionless shocks with anisotropic upstream pressure

Abstract

Shock waves are common in astrophysical environments. On many occasions, they are collisionless, which means they occur in settings where the mean free path is much larger than the dimensions of the system. For this very reason, magnetohydrodynamic (MHD) is not equipped to deal with such shocks, be it because it assumes binary collisions, hence temperature isotropy, when such isotropy is not guaranteed in the absence of collisions. Here we solve a model capable of dealing with perpendicular shocks with anisotropic upstream pressure. The system of MHD conservation equations is closed assuming the temperature normal to the flow is conserved at the crossing of the shock front. In the strong shock sonic limit, the behavior of a perpendicular shock with isotropic upstream is retrieved, regardless of the upstream anisotropy. Generally speaking, a rich variety of behaviors is found, inaccessible to MHD, depending on the upstream parameters. The present work can be viewed as the companion paper of MNRAS 520, 6083-6090 (2023), where the case of a parallel shock was treated. Differences and similarities with the present case are discussed.