Electron acceleration at quasi-perpendicular shocks in sub- and supercritical regimes: 2D and 3D simulations

Abstract

Shock accelerated electrons are found in many astrophysical environments, and the mechanisms by which they are accelerated to high energies are still not completely clear. For relatively high Mach numbers, the shock is supercritical, and its front exhibit broadband fluctuations, or ripples. Shock surface fluctuations have been object of many observational and theoretical studies, and are known to be important for electron acceleration. We employ a combination of hybrid Particle-In-Cell and test-particle methods to study how shock surface fluctuations influence the acceleration of suprathermal electrons in fully three dimensional simulations, and we give a complete comparison for the 2D and 3D cases. A range of different quasi-perpendicular shocks in 2D and 3D is examined, over a range of parameters compatible with the ones observed in the solar wind. Initial electron velocity distributions are taken as kappa functions, consistent with solar wind \emph{in-situ} measurements. Electron acceleration is found to be enhanced in the supercritical regime compared to subcritical. When the fully three-dimensional structure of the shock front is resolved, slightly larger energisation for the electrons is observed, and we suggest that this is due to the possibility for the electrons to interact with more than one surface fluctuation per interaction. In the supecritical regime, efficient electron energisation is found also at shock geometries departing from $\theta_{Bn}$ very close to 90$^\circ$. Two dimensional simulations show indications of unrealistic electron trapping, leading to slightly higher energisation in the subcritical cases.