Collisionless Shock As A Self-Regulatory System

Collisionless shocks are one of the most ubiquitous systems in space plasmas. They are one of the most efficient accelerators of charged particles. A collisionless shock is a multiscale system. The acceleration occurs at a scale that greatly exceeds the shock width. Yet, the processes in the whole shock affected space significantly depend on the structure of the shock transition region, in particular, on the ion dynamics inside the transition. The only in situ observations of collisionless shocks are possible only in the heliosphere. These observations show that the shock structure changes with the increase of the Mach number, becoming more and more complex. A collisionless shock is a self-organized system, the main task of which is the fast and stable transfer of the conserved quantities, that is, mass, momentum, and energy, from one side, upstream, to the other side, downstream, while adding entropy. "Fast" means that the transfer occurs at scales much smaller than the MHD scales. "Stable" means that there are not disruptions of substantial changes on average, except those which are caused by variations of ambient conditions. In this approach, the developing shock structure is the one that ensures this transfer. This means that if the transfer stability is not possible without an overshoot, an overshoot has to be formed. If it is not possible without rippling, rippling will develop. Since ions are the main carriers of these conserved quantities, it is ions that are responsible for developing the structure, and it is ions which have to most strongly affected by it. In particular, we show that overshoot plays an important role in regulating ion reflection so that the shock becomes stable.