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

Abstract

A pair of curved shocks in a collisionless plasma is examined with a two-dimensional particle-incell (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.

The interplay of the collisionless non-linear thin-shell instability with the ion acoustic instability

“…The size of the magnetic field patches we found here was comparable to those in the simulation by Dieckmann et al (2015b), where the curved shell was created by a spatial variation of the collision speed. The magnetic field in the present simulation and in that in (Dieckmann et al 2015b) damped out when the thin shell was destroyed by the proton-proton beam instability, evidencing that the hybrid structures were responsible for its growth.…”

“…The magnetic field in the present simulation and in that in (Dieckmann et al 2015b) damped out when the thin shell was destroyed by the proton-proton beam instability, evidencing that the hybrid structures were responsible for its growth.…”