Quasi-parallel Supercritical Shocks

Abstract

Quasi-parallel shocks are probably more common in nature than quasi-perpendicular. They exist for shock normal angles >45^∘. Their main distinctions from the latter are, first, their turbulent nature implying that the shock transition is less sharp and thus less well defined; second, the existence of an extended turbulent foreshock instead of a shock foot. This foreshock has distinct properties which are discussed. It consists of an electron and an ion foreshock. The main population is a diffuse ion component. The turbulence in the foreshock is generated by the (reflected and accelerated) foreshock particle populations. We discuss the various processes of wave generation referring to observations and sophisticated numerical simulations. An important point in quasi-parallel shock physics is the reformation of the shock which works completely differently from quasi-perpendicular shocks. Here it is provided by upstream low-frequency electromagnetic waves excited by the diffuse ion component. Steeping of these waves during shockward propagation and addition of the large amplitude waves at the shock transition reforms the shock front. The old shock front is expelled downstream where it causes downstream turbulence. During the reformation process the shock normal angle locally turns to low values. Thus the shock becomes locally quasi-perpendicular for the ions supporting particle reflection. Finally hot flow anomalies are briefly discussed.