  A thorough understanding of collisionless shocks requires knowledge of how
different ion species are accelerated across the shock. We investigate a bow
shock crossing using the Magnetospheric Multiscale spacecraft after a coronal
mass ejection crossed Earth, which led to solar wind consisting of protons,
alpha particles, and singly charge helium ions. The low Mach number of the bow
shock enabled the ions to be distinguished upstream and sometimes downstream of
the shock. Some of the protons are specularly reflected and produce
quasi-periodic fine structures in the velocity distribution functions
downstream of the shock. Heavier ions are shown to transit the shock without
reflection. However, the gyromotion of the heavier ions partially obscures the
fine structure of proton distributions. Additionally, the calculated proton
moments are unreliable when the different ion species are not distinguished by
the particle detector. The need to high time-resolution mass-resolving ion
detectors when investigating collisionless shocks is discussed.
