Ion Reflection by a Rippled Perpendicular Shock

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.