Magnetic Field Generation in Collisionless Shocks: Pattern Growth and Transport

We present results from three-dimensional particle simulations ofcollisionless shocks with relativistic counter-streaming ion-electron plasmas.Particles are followed over many skin depths downstream of the shock. Openboundaries allow the experiments to be continued for several particle crossingtimes. The experiments confirm the generation of strong magnetic and electricfields by a Weibel-like kinetic streaming instability, and demonstrate that theelectromagnetic fields propagate far downstream of the shock. The magneticfields are predominantly transversal, and are associated with merging ioncurrent channels. The total magnetic energy grows as the ion channels merge,and as the magnetic field patterns propagate down stream. The electronpopulations are quickly thermalized, while the ion populations retain distinctbulk speeds in shielded ion channels and thermalize much more slowly. Theseresults may help explain the origin of the magnetic fields responsible forafterglow synchrotron/jitter radiation from Gamma-Ray Bursts.Comment: 4 pages, 6 figures - Accepted to ApJL. Revised version following recommendations of referee report. Content reduced marginally. Conclusions unchange