Microphysics of Particle Reflection in Weibel-Mediated Shocks*

Particle-in-cell (PIC) simulations have shown that collisionless shocks mediated by the Weibel filamentation instability accelerate ~1% of incoming particles, while the majority are simply transmitted through the shock and become thermalized. The microphysical processes that determine whether an incoming particle will be transmitted or reflected are poorly understood. Here, we study the microphysics of particle reflection in Weibel-mediated shocks by tracking a shell of test particles in a PIC simulation. We find that electrons in positron-dominated filaments and positrons in electron-dominated filaments efficiently reflect off of strong magnetic features at the shock. We also find that while a significant fraction of incoming upstream particles experience an initial reflection at the shock, these reflected particles headed towards the upstream must then find filaments carrying the same direction of current in order to successfully move with the shock and participate in diffusive shock acceleration (DSA). We develop a model that predicts the percentage of reflected particles as a function of the properties of Weibel filamentation.