Magnetic Field Generation in Collisionless Shocks: Pattern Growth and Transport

We present results from three-dimensional particle simulations of collisionless shock formation, with relativistic counterstreaming ion-electron plasmas. Particles are followed over many skin depths downstream of the shock. Open boundaries allow the experiments to be continued for several particle crossing times. The experiments confirm the generation of strong magnetic and electric fields by a Weibel-like kinetic streaming instability and demonstrate that the electromagnetic fields propagate far downstream of the shock. The magnetic fields are predominantly transversal and are associated with merging ion current channels. The total magnetic energy grows as the ion channels merge and as the magnetic field patterns propagate downstream. The electron populations are quickly thermalized, while the ion populations retain distinct bulk speeds in shielded ion channels and thermalize much more slowly. The results help reveal processes of importance in collisionless shocks and may help to explain the origin of the magnetic fields responsible for afterglow synchrotron/jitter radiation from gamma-ray bursts.