Electron Preacceleration at Weak Quasi- Perpendicular ICM Shocks: Effects of Shock Surface Rippling

Abstract

Radio relics in the outskirts of galaxy clusters are interpreted as synchrotron radiation due to the relativistic electrons produced via diffusive shock acceleration (DSA) in shocks with low sonic Mach numbers, , in high . ICM plasma. Electron injection into the DSA process at such weak shocks is one of the key elements, which has yet to be fully understood. In this study, we explore the nature of kinetic microinstabilities excited in weak, quasi-perpendicular shocks through 2D Particle-in-Cell (PIC) simulations. We find Alfven-ion cyclotron (AIC), whistler, and mirror instabilities can be triggered by ion and electron temperature anisotropy in the immediate downstream of supercritical shocks with . In particular, AIC instability causes rippling of the shock surface, which in turn generates plasma waves on multi-scales and facilitates electron preacceleration. Our results may contribute to understanding the origins of radio relics.