Collisionless shocks in plasmas: when the density jump and the shock front do not obey MHD (at all)

According to MHD, a strong parallel shock is a few mean-free-paths thick, with a density jump of 4. The talk will comment on these 2 aspects for the case of a collisionless shock. First, I’ll review recent analytical works explaining how, as the plasma becomes collisionless, the front continuously switches from a few mean-free-path to a distance much shorter, in terms of the upstream plasma parameter. Second, I will also show how the density jump of a strong parallel collisionless shock can shrink to 2 instead of 4. This is due to the ability of a collisionless plasma to sustain a stable anisotropy in the presence of a magnetic field. PIC simulations will be presented confirming this conclusion.