Hybrid simulations of collisionless shocks in exponential density gradients

We present simulations of a strong 3-D collisionless shock transitioning into sub-Alfv\'{e}nic waves. The transition occurs due changes in the background plasma parameters. The Alfv\'{e}n speed eventually exceeds the shock speed as the background plasma density falls with altitude; the shock does not run out of energy. At this velocity transition, the shock disassembles into two types of waves: the usual compressional Alfv\'{e}n wave and a left-hand polarized electromagnetic shear Alfv\'{e}n wave. This later wave shows remarkable 3-D coherence. Analysis suggests that there are two possible sources of energy: (1) coupling to the strong electromagnetic waves that exist within the collisionless shock and (2) the density and magnetic field gradients at the interface. Results from recent simulations using our three dimensional parallel hybrid plasma code will be presented and discussed.