# Weibel-Mediated Shocks Propagating into Inhomogeneous Electron-Positron Plasmas

## TL;DR

This study uses PIC simulations to show that inhomogeneous upstream plasmas cause collisionless shocks to sustain stronger magnetic fields downstream, explaining observed GRB afterglow features.

## Contribution

It demonstrates that upstream density inhomogeneities significantly influence magnetic field strength in collisionless shocks, a novel insight into shock physics.

## Key findings

- Magnetic fields decay more slowly in inhomogeneous media.
- Sound and entropy waves are generated by shock interactions.
- Temperature anisotropy can account for observed magnetic field strengths.

## Abstract

The external forward shock emitting the gamma-ray burst (GRB) afterglow is collisionless, and it is mediated by the Weibel instability which generates the magnetic field. The GRB afterglow shows that the magnetic field in the large downstream region is much stronger than the shock-compressed pre-shock field. However, particle-in-cell (PIC) simulations of relativistic shocks propagating into homogeneous media show that the Weibel generated field decays near the shock front. Some GRB observations and theoretical studies suggest that the preshock medium is inhomogeneous. We perform the PIC simulation of a relativistic shock propagating into inhomogeneous plasma. It is found that the post-shock magnetic field decays slowly compared with the homogeneous case. Sound waves and entropy waves are also generated by the shock-wave interaction, and temperature anisotropy is produced by the sound wave in the downstream region. The free energy of the temperature anisotropy is large enough to explain the observed field strength. Our results show that the upstream density fluctuation has a significant effect in the downstream region of collisionless shocks even if the wavelength of the upstream inhomogeneity is much larger than the kinetic scale.