Relativistic Unmagnetized Collisionless Shocks Propagating into the Inhomogeneous Media

Abstract

The Weibel instability generates magnetic fields in a collisionless plasma with anisotropic temper- ature, which is thought to be a crucial role for particle acceleration and magnetic field generation in relativistic collisionless shocks. According to observations of afterglows of Gamma-ray Busts (GRBs), magnetic fields are strongly amplified to about 100 times the shock-compressed value in the large downstream region of the relativistic shock. However, recent simulations of colli- sionless shocks in homogeneous plasmas suggest that the magnetic field generated by the Weibel instability decays rapidly, which cannot explain observed properties of afterglows of GRBs. In reality, there must be density fluctuations. We proposed a new model for the magnetic field gen- eration in the far downstream region of the relativistic shock. Relativistic shocks propagating to inhomogeneous plasmas make an anisotropic density structure in the downstream region. Then, an anisotropic velocity distribution is generated, so that the magnetic field is generated by the Weibel instability in the far downstream region (S. Tomita & Y. Ohira, ApJ, 2016). In this study, we perform Particle-In-Cell (PIC) simulations of relativistic unmagnetized collisionless shocks propagating into the inhomogeneous plasma to investigate whether or not our model actually work. The simulation shows that there is a larger temperature anisotropy in the far downstream region compared with a uniform case. The observed temperature anisotropy is sufficiently large to generate the required magnetic field. In oder to see the Weibel instability by the observed anisotropy, we need larger simulations.