Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas

TL;DR

This study uses large-scale kinetic simulations to show that shock rippling significantly enhances electron acceleration in high-beta, low-Mach-number collisionless shocks, producing non-thermal electron spectra consistent with observations.

Contribution

It demonstrates that shock rippling and multi-scale turbulence critically boost electron acceleration and produce realistic power-law spectra in high-beta plasmas.

Findings

Electron acceleration rate increases with shock rippling.

Downstream electron spectrum follows a power-law with index ~2.5.

Electrons reach energies suitable for diffusive shock acceleration.

Abstract

Using large-scale fully-kinetic two-dimensional particle-in-cell simulations, we investigate the effects of shock rippling on electron acceleration at low-Mach-number shocks propagating in high-$\beta$ plasmas, in application to merger shocks in galaxy clusters. We find that the electron acceleration rate increases considerably when the rippling modes appear. The main acceleration mechanism is stochastic shock-drift acceleration, in which electrons are confined at the shock by pitch-angle scattering off turbulence and gain energy from the motional electric field. The presence of multi-scale magnetic turbulence at the shock transition and the region immediately behind the main shock overshoot is essential for electron energization. Wide-energy non-thermal electron distributions are formed both upstream and downstream of the shock. The maximum energy of the electrons is sufficient for their injection into diffusive shock acceleration. We show for the first time that the downstream electron spectrum has a~power-law form with index $p\approx 2.5$, in agreement with observations.