Electron shock surfing acceleration in multidimensions: Two-dimensional particle-in-cell simulation of collisionless perpendicular shock

Abstract

Electron acceleration mechanisms in high-Mach-number collisionless shocks propagating in a weakly magnetized medium is investigated using a self-consistent two-dimensional particle-in-cell simulation. Simulation results show that strong electrostatic waves are excited via the electron-ion electrostatic two-stream instability at the leading edge of the shock transition region as in the case of earlier one-dimensional simulations. We observe strong electron acceleration that is associated with the turbulent electrostatic waves in the shock transition region. The electron energy spectrum in the shock transition region exhibits a clear power-law distribution with spectral index of 2.0-2.5. By analyzing the trajectories of accelerated electrons, we find that the acceleration mechanism is very similar to shock-surfing acceleration of ions. In contrast to the ion shock surfing, however, the energetic electrons are reflected by electron-scale electrostatic fluctuations in the shock transition region and not by the ion-scale cross-shock electrostatic potential. The reflected electrons are then accelerated by the convective electric field in front of the shock. We conclude that the multidimensional effects as well as the self-consistent shock structure are essential for the strong electron acceleration at high-Mach-number shocks. © 2009. The American Astronomical Society. All rights reserved.