Weibel instability driven by spatially anisotropic density structures

TL;DR

This study demonstrates through simulations that spatially anisotropic density structures can sustain large magnetic fields via the Weibel instability, potentially explaining gamma-ray burst afterglow emissions.

Contribution

It introduces the role of anisotropic density structures in prolonging magnetic field generation by the Weibel instability in relativistic shocks.

Findings

Large magnetic fields are maintained longer with anisotropic density structures.

Particles escape anisotropically, generating temperature anisotropy.

Results suggest sufficient magnetic field coverage for GRB afterglow emission.

Abstract

Observations of afterglows of gamma-ray bursts suggest (GRBs) that post-shock magnetic fields are strongly amplified to about 100 times the shock-compressed value. The Weibel instability appears to play an important role in generating of the magnetic field. However, recent simulations of collisionless shocks in homogeneous plasmas show that the magnetic field generated by the Weibel instability rapidly decays. There must be some density fluctuations in interstellar and circumstellar media. The density fluctuations are anisotropically compressed in the downstream region of relativistic shocks. In this paper, we study the Weibel instability in electron--positron plasmas with the spatially anisotropic density distributions by means of two-dimensional particle-in-cell simulations. We find that large magnetic fields are maintained for a longer time by the Weibel instability driven by the spatially anisotropic density structure. Particles anisotropically escape from the high density region, so that the temperature anisotropy is generated and the Weibel instability becomes unstable. Our simulation results suggest that the Weibel instability driven by an anisotropic density structure can generate sufficiently large magnetic fields and they can cover sufficiently large regions to explain the afterglow emission of GRBs.