Electron Beams Generated by the Electron Kelvin-Helmholtz Instability at a Quasi-Perpendicular Shock

Electron beams are considered to be important free energy sources for the excitation of various plasma waves at quasi-perpendicular shocks. In this article, we perform a two-dimensional particle-in-cell simulation of a low-plasma-β quasi-perpendicular shock. The magnetic field at the shock ramp has a large gradient, where upstream electrons and ions are separated due to their different gyro-radius. This charge separation induces a sub-ion scale electric field at the shock ramp. The electric drift of electrons in this field can induce an electron shear flow, resulting in the excitation of Kelvin-Helmholtz instability and the generation of electron vortices. These vortices further cause charge separation at their centers, resulting in a large electrostatic field. The electrons trapped in these vortices can gain energy from the parallel component of the electric field, which eventually leads to field-aligned electron beams. Our results provide a novel process for generating electron beams at low-plasma-β quasi-perpendicular shocks.