  The electromagnetic emission from neutron star mergers is comprised of
multiple components. Synchrotron emission from the disk-powered jet as well as
thermal emission from the merger ejecta (powered by a variety of sources) are
among the most studied sources. The low masses and high velocities of the
merger ejecta quickly develop conditions where emission from collisionless
shocks becomes critical and synchrotron emission from the merger ejecta
constitutes a third component to the observed signal. The aim of this project
is to examine shock development, magnetic field generation and particle
acceleration in the case of mildly relativistic shocks, which are expected when
the tidal ejecta of neutron star mergers drive a shock into the external
medium. Using LANL's VPIC (vector particle-in-cell) code, we have run
simulations of such mildly-relativistic, collisionless, weakly-magnetized
plasmas and compute the resultant magnetic fields and particle energy spectra.
We show the effects of varying plasma conditions, as well as explore the
validity of using different proton to electron mass ratios in VPIC. Our results
have implications for observing late-time electromagnetic counterparts to
gravitational wave detections of neutron star mergers.
