  We study relativistic unmagnetized collisionless shocks using unprecedentedly
large particle-in-cell simulations of two-dimensional pair plasma. High energy
particles accelerated by the shock are found to drive magnetic field evolution
on a timescale >10^4 plasma times. Progressively stronger magnetic fields are
generated on larger scales in a growing region around the shock.
Shock-generated magnetic fields and accelerated particles carry >1% and >10% of
the downstream energy flux, respectively. Our results suggest limits on the
magnetization of relativistic astrophysical flows.
