The evolution of a slow electrostatic shock into a plasma shock mediated by electrostatic turbulence

TL;DR

This study uses 2D PIC simulations to investigate how a slow electrostatic shock evolves into a plasma shock mediated by turbulence, relevant to astrophysical and laboratory plasma conditions.

Contribution

It demonstrates the transition process of a slow electrostatic shock into a turbulence-mediated plasma shock in a collision of plasma clouds with different ion temperatures.

Findings

Electrostatic ion acoustic turbulence develops ahead of the shock.

The shock transition layer widens and ions thermalize due to turbulence.

The forward shock remains stable despite turbulence development.

Abstract

The collision of two plasma clouds at a speed that exceeds the ion acoustic speed can result in the formation of shocks. This phenomenon is observed not only in astrophysical scenarios such as the propagation of supernova remnant (SNR) blast shells into the interstellar medium, but also in laboratory-based laser-plasma experiments. These experiments and supporting simulations are thus seen as an attractive platform for the small-scale reproduction and study of astrophysical shocks in the laboratory. We model two plasma clouds, which consist of electrons and ions, with a 2D PIC simulation. The ion temperatures of both clouds differ by a factor of 10. Both clouds collide at a speed, which is realistic for laboratory studies and for SNR shocks in their late evolution phase like that of RCW86. A magnetic field, which is orthogonal to the simulation plane, has a strength that is comparable to that at SNR shocks. A forward shock forms between the overlap layer of both plasma clouds and the cloud with the cooler ions. A large-amplitude ion acoustic wave is observed between the overlap layer and the cloud with the hotter ions. It does not steepen into a reverse shock, because its speed is below the ion acoustic speed. A gradient of the magnetic field amplitude builds up close to the forward shock as it compresses the magnetic field. This gradient gives rise to an electron drift that is fast enough to trigger an instability. Electrostatic ion acoustic wave turbulence develops ahead of the shock. It widens its transition layer and thermalizes the ions, but the forward shock remains intact.