Acceleration Mechanics in Relativistic Shocks by the Weibel Instability

Abstract

Using a 3-D relativistic PIC code, we have investigated particle acceleration associated with relativistic electron-ion or electron-positron jets with a longer simulation system than in order to investigate the nonlinear stage of the Weibel instability and its particle acceleration mechanism. The current channels generated by the Weibel instability are surrounded by toroidal magnetic fields and radial electric fields. The component of the Poynting vector parallel to the jet direction (E x B}_z becomes positive in a large region along the jet propagation direction in the rest frame of the ambient plasma, accelerating the ambient plasma. E x B forces do accelerate some jet particles to higher energies in the parallel direction in addition to acceleration in the perpendicular directions relative to the initial motion. In particular, for the electron-ion case the large scale current channels generated by the ion Weibel instability lead to more acceleration. The accelerated jet electrons in the electron-ion jet have a significant non-thermal bump and high energy tail. However, in the electron-positron case, accelerated jet electrons do not have an obvious non-thermal component to the high energy tail. In the electron-positron case, initial acceleration occurs as current channels form and then continues at a much lesser rate as the current channels and corresponding toroidal magnetic fields generated by the Weibel instability dissipate.