Electron Shock Surfing Acceleration in Multidimensions: Two-dimensional Particle-In-Cell Simulation of Collisionless Perpendicular Shock

TL;DR

This study uses two-dimensional particle-in-cell simulations to reveal how electrons are accelerated at high Mach number collisionless shocks, highlighting the importance of multidimensional effects and electrostatic turbulence in the acceleration process.

Contribution

It demonstrates that multidimensional electrostatic turbulence enables strong electron acceleration via shock surfing, a mechanism previously studied mainly in one dimension.

Findings

Electrostatic waves are excited at the shock transition region.

Electrons exhibit a power-law energy spectrum with index 2.0-2.5.

Electron acceleration is similar to shock surfing but involves electron-scale fluctuations.

Abstract

Electron acceleration mechanism at high Mach number collisionless shocks propagating in a weakly magnetized medium is investigated by 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 $2.0 {\rm -} 2.5$. 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, but 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.