Bridging the Gap between Collisional and Collisionless Plasma Shocks: A Simulation Study using OSIRIS

TL;DR

This study uses particle-in-cell simulations with a Coulomb-collision module to explore the transition between collisional and collisionless plasma shocks, revealing a smooth change in shock width around the ion plasma parameter N_D ≈ 1.

Contribution

It introduces a simulation framework that bridges collisional and collisionless shock regimes, demonstrating a continuous transition in shock width governed by the ion plasma parameter.

Findings

Shock width transitions smoothly near N_D ≈ 1

Results align with Mott-Smith and Tidman's theories in respective regimes

Ion plasma parameter effectively indicates shock regime change

Abstract

Shock waves in plasmas can be characterized by the mechanisms behind their formation. When binary collisions are frequent, dissipation is collision-driven and the shock width is a few mean free paths. In contrast, collisionless shocks rely on collective plasma processes to establish dissipation on scales well below the mean free path. We bridge these regimes with particle-in-cell simulations using OSIRIS with a Coulomb-collision module, varying parameters that control collisionality. We find a smooth transition of the shock width in the intermediate region where the ion plasma parameter $N_D \approx 1$. Our results recover the asymptotic predictions: a collisional-regime width consistent with the Mott-Smith ansatz with a BGK operator, and the collisionless limit consistent with Tidman's classical formalism. The ion plasma parameter thus serves as a practical metric for identifying when shocks shift from fluid-like, mean-free-path scales to collisionless, sub-mean-free-path scales. We discuss implications for astrophysical environments, where shock breakout changes the shock width and marks the onset of efficient particle acceleration.