- Abstract Ion heating in quasi-perpendicular collisionless shocks is due to ion gyration just behind the ramp. The gyrating distribution gyrotropizes due to kinematic gyrophase mixing. This collisionless relaxation occurs in high Mach number and low Mach number shocks as well. In low Mach number low beta shocks the relaxation is slow and the downstream ion pressure oscillates in space. The pressure oscillations cause magnetic field oscillations which should be observed as a periodic series of overshoots and undershoots.