On the structure of relativistic collisionless shocks in electron-ion plasmas

TL;DR

This paper uses particle-in-cell simulations to study unmagnetized relativistic electron-ion shocks, revealing ion Weibel instability as a key mediator and electron heating mechanisms relevant to Gamma-Ray Burst afterglows.

Contribution

It provides the first long-term 2.5D simulations of relativistic electron-ion shocks at high mass ratios, demonstrating shock formation and electron-ion energy equipartition.

Findings

Ion Weibel instability mediates shocks at high mass ratios.

Electron heating occurs via interaction with ion filaments.

Postshock flow reaches near equipartition between ions and electrons.

Abstract

Relativistic collisionless shocks in electron-ion plasma are thought to occur in the afterglow phase of Gamma-Ray Bursts (GRBs), and in other environments where relativistic flows interact with the interstellar medium. A particular regime of shocks in an unmagnetized plasma has generated much interest for GRB applications. In this paper we present ab-initio particle-in-cell simulations of unmagnetized relativistic electron-ion shocks. Using long-term 2.5-dimensional simulations with ion-electron mass ratios from 16 to 1000 we resolve the shock formation and reach a steady-state shock structure beyond the initial transient. We find that even at high ion-electron mass ratios initially unmagnetized shocks can be effectively mediated by the ion Weibel instability with a typical shock thickness of ~50 ion skin-depths. Upstream of the shock the interaction with merging ion current filaments heats the electron component, so that the postshock flow achieves near equipartition between the ions and electrons, with electron temperature reaching 50% of the ion temperature. This energy exchange helps to explain the large electron energy fraction inferred from GRB afterglow observations.