Collisionless ion-electron energy exchange in magnetized astrophysical shocks*

Magnetized collisionless shocks are ubiquitous in the universe, and energy partition in these shocks has long been an open question. Independent thermalization would predict a largely uneven energy partition because of the large ion-electron mass ratio, contradicting astronomical observations. Our kinetic simulations of low Mach number magnetized collisionless shocks show a significant energy exchange between ions and electrons in the downstream of the shocks, which implies a collisionless electron heating mechanism. A multi-fluid model indicates a resonance between electron whistler and ion magnetohydrodynamic waves may provide new mechanisms that account for the energy transfer from ions to electrons.