  Astrophysical shocks are often studied in the high Mach number limit but
weakly compressive fast shocks can occur in magnetic reconnection outflows and
are considered to be a site of particle energization in solar flares. Here we
study the microphysics of such perpendicular, low Mach number collisionless
shocks using two-dimensional particle-in-cell (PIC) simulations with a reduced
ion/electron mass ratio and employ a moving wall boundary method for initial
generation of the shock. This moving wall method allows for more control of the
shock speed, smaller simulation box sizes, and longer simulation times than the
commonly used fixed wall, reflection method of shock formation. Our results,
which are independent of the shock formation method, reveal the prevalence
shock drift acceleration (SDA) of both electron and ions in a purely
perpendicular shock with Alfv\'en Mach number $M_A=6.8$ and ratio of thermal to
magnetic pressure $\beta=8$. We determine the respective minimum energies
required for electrons and ions to incur SDA. We derive a theoretical electron
distribution via SDA that compares to the simulation results. We also show that
a modified two-stream instability due to the incoming and reflecting ions in
the shock transition region acts as the mechanism to generate collisionless
plasma turbulence that sustains the shock.
