Bridging the gap between collisional and collisionless plasma shocks: A simulation study using OSIRIS

Shock waves in plasma can be characterized according to the physical mechanisms behind their formation. When collisions are frequent, shock dissipation is mainly binary-collision driven, and the shock width (SW) is often comparable to a few mean-free-paths. In contrast, collisionless shocks rely on collective plasma processes to establish dissipation on scales far below the mean-free-path. The purpose of this study is to bridge these two regimes. Here, we present simulations using the OSIRIS particle-in-cell code with the Coulomb collision module to explore the gradual transition between collisional and collisionless shocks by varying plasma parameters that determine collisionality. We show a smooth transition of the SW in the intermediate region, where the ion plasma parameter is ND≈1. Our numerical results confirm earlier theoretical predictions from existing theories in the asymptotic regimes, where the Mott–Smith ansatz, combined with a full Bhatnagar–Gross–Krook (BGK) collision operator, was used to obtain the collisional-regime SW, while the classical Tidman formalism describes the collisionless limit. We demonstrate that the ion plasma parameter provides a useful metric for identifying when shocks transition from a fluid-like, mean-free-path scale to a collisionless, sub-mean-free-path scale. We discuss the importance of our results in astrophysical environments, where shock breakout causes a transition in the SW and marks the beginning of the acceleration of particles to high energies.