Microphysics of Particle Reflection in Weibel-mediated Shocks

Particle-in-cell (PIC) simulations have shown that relativistic collisionless shocks mediated by the Weibel instability accelerate ∼1% of incoming particles, while the majority are transmitted through the shock and become thermalized. The microphysical processes that determine whether an incoming particle will be transmitted or reflected are poorly understood. We study the microphysics of particle reflection in Weibel-mediated shocks by tracking a shell of test particles in a PIC simulation of a shock in pair plasma. We find that electrons in positron-dominated filaments and positrons in electron-dominated filaments efficiently reflect off of strong magnetic structures at the shock. To participate in diffusive shock acceleration, however, these reflected particles headed toward the upstream must avoid getting advected downstream. This is enabled by incoming filaments, which trap reflected particles carrying the same sign of current as the filaments. The final injection efficiency on the order of ∼1% thus results from the effectiveness of the initial reflection at the shock and the reflected particles’ probability of survival in the upstream postreflection. We develop a model that predicts the fraction of high-energy particles as a function of the properties of Weibel filamentation.