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

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.