Electron and ion thermalization in collisionless shocks

Astrophysical collisionless shocks play a central role in magnetic field amplification, plasma heating, and acceleration of galactic cosmic rays. A fundamental open question is what are the mechanisms that control the energy partition between ions and electrons downstream of the shock. Observations of high Mach number shocks have shown that T_e/T_i ≥ 0.1, indicating that electrons acquire significantly more energy than that corresponding to simple thermalization of their initial kinetic energy. In this work we use 3D Particle-In-Cell simulations to investigate the electron and ion heating mechanisms in high Mach number collisionless shocks, and find that T_e/T_i is dictated by the nonlinear interplay between the ion current-filamentation and the drift-kink electromagnetic instabilities. The simulations yield temperature ratios compatible with astrophysical observations as well as with recent laser-produced collisionless shock experiments.