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

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 multiscale 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 approximate to 2.5, in agreement with observations.

Automated summary

Using large-scale fully kinetic two-dimensional particle-in-cell simulations, the effects of shock rippling on electron acceleration at low-Mach-number shocks propagating in high-beta plasmas were investigated. It was found that the electron-acceleration rate increases considerably when the rippling modes appear, with the main acceleration mechanism being stochastic shock-drift acceleration. Multiscale magnetic turbulence at the shock transition and the region behind the main shock overshoot are essential for electron energization, leading to wide-energy non-thermal electron distributions formed both upstream and downstream of the shock. The downstream electron spectrum was shown for the first time to have a power-law form with index p approximate to 2.5, consistent with observations.