We discuss 3D simulations of relativistic collisionless shocks in
electron-positron pair plasmas using the particle-in-cell (PIC) method. The
shock structure is mainly controlled by the shock's magnetization ("sigma"
parameter). We demonstrate how the structure of the shock varies as a function
of sigma for perpendicular shocks. At low magnetizations the shock is mediated
mainly by the Weibel instability which generates transient magnetic fields that
can exceed the initial field. At larger magnetizations the shock is dominated
by magnetic reflections. We demonstrate where the transition occurs and argue
that it is impossible to have very low magnetization collisionless shocks in
nature (in more than one spatial dimension). We further discuss the
acceleration properties of these shocks, and show that higher magnetization
perpendicular shocks do not efficiently accelerate nonthermal particles in 3D.
Among other astrophysical applications, this may pose a restriction on the
structure and composition of gamma-ray bursts and pulsar wind outflows.Comment: 6 pages, invited talk at "Astrophysical Sources of High Energy
Particles and Radiation," Torun, June 20 - 24, 200
