Foreshock shock interaction at collisionless quasi-parallel shocks

A unique feature of oblique collisionless shocks is the existence of a foreshock, a region upstream of the shock which contains waves and particles produced directly, and indirectly, by the shock. Because of the supersonic flow in the upstream region an important population of waves can be convected into the shock itself. The possible interaction between the foreshock and the shock has long been admitted, but recently some attempts have been made to investigate the processes and effects of this interaction. The following topics are addressed: The effect of upstream waves on the local shock angle; the control of ion reflection and sharp field gradients by upstream waves; the interaction of cold reflected ion beams and upstream waves; the growth of upstream waves into large amplitude structures at the shock transition; the role of energetic particles. A summary is given of the various suggested types of interaction, and their relevance to thermalization at the shock.