Effects of shock and turbulence properties on electron acceleration

TL;DR

This study uses test particle simulations to analyze how shock and turbulence properties influence electron acceleration efficiency at collisionless shocks, revealing dependencies on shock angle, turbulence level, and shock thickness.

Contribution

It provides new insights into the roles of turbulence level and shock thickness in electron acceleration at different shock geometries.

Findings

Perpendicular shocks with low turbulence enhance electron acceleration via SDA.

Parallel shocks with high turbulence promote strong Fermi acceleration.

Shock thickness affects acceleration efficiency, with a critical bend-over thickness.

Abstract

Using test particle simulations we study electron acceleration at collisionless shocks with a two-component model turbulent magnetic field with slab component including dissipation range. We investigate the importance of shock normal angle $\theta_{Bn}$, magnetic turbulence level $\left(b/B_0\right)^2$, and shock thickness on the acceleration efficiency of electrons. It is shown that at perpendicular shocks the electron acceleration efficiency is enhanced with the decreasing of $\left(b/B_0\right)^2$, and at $\left(b/B_0\right)^2=0.01$ the acceleration becomes significant due to strong drift electric field with long time particles staying near the shock front for shock drift acceleration (SDA). In addition, at parallel shocks the electron acceleration efficiency is increasing with the increasing of $\left(b/B_0\right)^2$, and at $\left(b/B_0\right)^2=10.0$ the acceleration is very strong due to sufficient pitch-angle scattering for first-order Fermi acceleration, as well as due to large local component of magnetic field perpendicular to shock normal angle for SDA. On the other hand, the high perpendicular shock acceleration with $\left(b/B_0\right)^2=0.01$ is stronger than the high parallel shock acceleration with ($\left(b/B_0\right)^2=10.0$), the reason might be the assumption that SDA is more efficient than first-order Fermi acceleration. Furthermore, for oblique shocks, the acceleration efficiency is small no matter the turbulence level is low or high. Moreover, for the effect of shock thickness on electron acceleration at perpendicular shocks, we show that there exists the bend-over thickness, $L_{\text{diff,b}}$. The acceleration efficiency does not change evidently if the shock thickness is much smaller than $L_{\text{diff,b}}$. However, if the shock thickness is much larger than $L_{\text{diff,b}}$, the acceleration efficiency starts to drop abruptly.