Electron Pre-Acceleration at Nonrelativistic High-Mach-Number Perpendicular Shocks

TL;DR

This study uses particle-in-cell simulations to investigate electron pre-acceleration mechanisms at nonrelativistic perpendicular shocks, revealing how shock reformation, magnetic field orientation, and plasma beta influence electron energization relevant to supernova remnants.

Contribution

The paper provides new insights into electron pre-acceleration processes at nonrelativistic shocks, highlighting the effects of magnetic field orientation and plasma beta on acceleration efficiency.

Findings

Electron energization occurs via shock-surfing acceleration (SSA).

Magnetic field orientation significantly affects the fraction of supra-thermal electrons.

Moderate plasma beta enhances pre-acceleration efficiency.

Abstract

We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta $\beta_{\rm p}=5\cdot 10^{-4}$ and $\beta_{\rm p}=0.5$), we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane while keeping it at $90^\circ$ to the shock normal. Ion reflection off the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration (SSA) mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field the fraction of supra-thermal electrons is much higher than for other configurations, because only in this case the Buneman modes are fully captured by the 2D simulation grid. Shocks in plasma with moderate $\beta_{\rm p}$ provide more efficient pre-acceleration. The relevance of our results to the physics of fully three-dimensional systems is discussed.