Perpendicular relativistic shocks in magnetized pair plasma

TL;DR

This study uses 2D Particle-in-Cell simulations to explore how perpendicular relativistic shocks in magnetized pair plasmas transition from Weibel-mediated to magnetic-reflection-shaped structures, revealing effects on particle acceleration and shock properties.

Contribution

It provides a comprehensive simulation-based analysis of shock transition regimes, particle acceleration mechanisms, and the impact of magnetization on shock structure and dynamics.

Findings

Transition occurs at $10^{-3}<\sigma<10^{-2}$ with strong precursor currents.

Particle acceleration is efficient at low magnetizations and suppressed at high magnetizations.

Diffusive Shock Acceleration is only observed in weakly magnetized shocks.

Abstract

Perpendicular relativistic ($\gamma_0=10$) shocks in magnetized pair plasmas are investigated using two dimensional Particle-in-Cell simulations. A systematic survey, from unmagnetized to strongly magnetized shocks, is presented accurately capturing the transition from Weibel-mediated to magnetic-reflection-shaped shocks. This transition is found to occur for upstream flow magnetizations $10^{-3}<\sigma<10^{-2}$ at which a strong perpendicular net current is observed in the precursor, driving the so-called current-filamentation instability. The global structure of the shock and shock formation time are discussed. The MHD shock jump conditions are found in good agreement with the numerical results, except for $10^{-4} < \sigma < 10^{-2}$ where a deviation up to 10\% is observed. The particle precursor length converges toward the Larmor radius of particles injected in the upstream magnetic field at intermediate magnetizations. For $\sigma>10^{-2}$, it leaves place to a purely electromagnetic precursor following from the strong emission of electromagnetic waves at the shock front. Particle acceleration is found to be efficient in weakly magnetized perpendicular shocks in agreement with previous works, and is fully suppressed for $\sigma > 10^{-2}$. Diffusive Shock Acceleration is observed only in weakly magnetized shocks, while a dominant contribution of Shock Drift Acceleration is evidenced at intermediate magnetizations. The spatial diffusion coefficients are extracted from the simulations allowing for a deeper insight into the self-consistent particle kinematics and scale with the square of the particle energy in weakly magnetized shocks. These results have implications for particle acceleration in the internal shocks of AGN jets and in the termination shocks of Pulsar Wind Nebulae.