Quasi-parallel collisionless shocks: revealing the nature of cosmic particle accelerators

Abstract

These shocks propagate through completely or partially ionized medium which is embedded in an ambient magnetic field. They are thought to be the most common source of the highest energy particles in a cosmic plasma. We explain the nature of quasi-parallel collisionless shocks and consequential particle acceleration. The plasma particles that are stricken by the shock front, become energized and gain a chaotic-like behaviour which dissorders the plasma behind the shock. In contrast to their tendency to increase the plasma entropy, shocks are governed by highly coherent and finely tuned, self-consistent plasma processes. By the means of particle-in-cell and test particle simulations, and linear plasma theory, we enter the microworld of collisionless shocks. We explain how quasi-parallel shocks are triggered and mediated by resonant-like electromagnetic micro-instabilities. We give a short review on the shock reformation process and present a new physical model. Finally, we explain the mechanisms of ion and electron thermalization and acceleration at the nature leading-edge particle accelerators.