  We use multi-spacecraft Magnetospheric Multiscale (MMS) observations to
investigate electric fields and ion reflection at a non-stationary
collisionless perpendicular plasma shock. We identify sub-proton scale (5-10
electron inertial lengths) large-amplitude normal electric fields, balanced by
the Hall term ($\mathbf{J} \times \mathbf{B}/ne$), as a transient feature of
the shock ramp related to non-stationarity (rippling). The associated
electrostatic potential, comparable to the energy of the incident solar wind
protons, decelerates incident ions and reflects a significant fraction of
protons, resulting in more efficient shock-drift acceleration than a stationary
planar shock.
