Bridging the gap between collisional and collisionless shock waves

TL;DR

This paper investigates the transition between collisional and collisionless shock waves in plasmas, providing a parameterization that explains how shock front thickness varies across regimes, relevant for astrophysical phenomena.

Contribution

It introduces a unified framework to describe the transition between collisional and collisionless shocks using the upstream plasma parameter Λ, bridging the gap between two regimes.

Findings

Transition occurs at Λ ≈ 1.12, marking a change in shock front thickness.

Shock front thickness scales with Mach number and mean-free-path in collisional regime.

In astrophysical settings, the shock front can be much thinner than the mean-free-path.

Abstract

While the front of a fluid shock is a few mean-free-paths thick, the front of a collisionless shock can be orders of magnitude thinner. By bridging between a collisional and a collisionless formalism, we assess the transition between these two regimes. We consider non-relativistic, un-magnetized, planar shocks in electron/ion plasmas. In addition, our treatment of the collisionless regime is restricted to high Mach number electrostatic shocks. We find that the transition can be parameterized by the upstream plasma parameter $\Lambda$ which measures the coupling of the upstream medium. For $\Lambda \lesssim 1.12$, the upstream is collisional, i.e. strongly coupled, and the strong shock front is about $\mathcal{M}_1 \lambda_{\mathrm{mfp},1}$ thick, where $\lambda_{\mathrm{mfp},1}$ and $\mathcal{M}_1 $ are the upstream mean-free-path and Mach number respectively. A transition occurs for $\Lambda \sim 1.12$ beyond which the front is $\sim \mathcal{M}_1\lambda_{\mathrm{mfp},1}\ln \Lambda/\Lambda$ thick for $\Lambda\gtrsim 1.12$. Considering $\Lambda$ can reach billions in astrophysical settings, this allows to understand how the front of a collisionless shock can be orders of magnitude smaller than the mean-free-path, and how physics transitions continuously between these 2 extremes.