A magnetized, spherical plasma expansion in an inhomogeneous plasma: Transition from super‐ to sub‐Alfvénic

In this letter the transition of a strong 3‐D collisionless shock into sub‐Alfvénic waves is numerically. The transition occurs due to changes in the background plasma parameters. The Alfvén speed eventually exceeds the shock speed; the shock does not run out of energy. At this velocity transition, the shock disassembles into two types of waves: the usual compressional Alfvén wave and a left‐hand polarized electromagnetic shear Alfvén wave. This latter wave shows remarkable 3‐D coherence, and preliminary analysis suggests that there are two possible sources of energy: one is coupling to the strong electromagnetic waves that exist within the collisionless shock and the other is the density and magnetic field gradients at the interface.