Shocks in unmagnetized plasma with a shear flow: Stability and magnetic field generation

TL;DR

This study uses particle-in-cell simulations to analyze the stability and magnetic field generation in curved shocks within unmagnetized, collisionless plasma, revealing the development of coherent magnetic patches due to electron flow disalignment.

Contribution

It demonstrates the stability of curved shocks and the mechanism of magnetic field generation through electron flow disalignment in unmagnetized plasma.

Findings

Shocks remain stable for tens of inverse ion plasma frequencies.

Magnetic patches grow due to rotational electron flows.

Shock curvature increases with time due to collision speed variation.

Abstract

A pair of curved shocks in a collisionless plasma is examined with a two-dimensional particle-in-cell (PIC) simulation. The shocks are created by the collision of two electron-ion clouds at a speed that exceeds everywhere the threshold speed for shock formation. A variation of the collision speed along the initially planar collision boundary, which is comparable to the ion acoustic speed, yields a curvature of the shock that increases with time. The spatially varying Mach number of the shocks results in a variation of the downstream density in the direction along the shock boundary. This variation is eventually equilibrated by the thermal diffusion of ions. The pair of shocks is stable for tens of inverse ion plasma frequencies. The angle between the mean flow velocity vector of the inflowing upstream plasma and the shock's electrostatic field increases steadily during this time. The disalignment of both vectors gives rise to a rotational electron flow, which yields the growth of magnetic field patches that are coherent over tens of electron skin depths.