  We present numerical results from plasma particle simulations of
collisionless shocks and ultra-relativistic counter-streaming plasmas. We
demonstrate how the field-particle interactions lead to particle acceleration
behind the shock-front. Further, we demonstrate how ultra relativistic
counter-streaming plasmas create large scale patchy magnetic field structures
and that these field structures propagate down-stream of the shock front. These
results may help explain the origin of the magnetic fields and accelerated
electrons responsible for afterglow synchrotron radiation from gamma ray
bursts.
