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The unprecedented detail of measurements by the four Magnetospheric Multiscale (MMS) spacecraft enable deeper investigation of quasi-perpendicular collisionless shocks. We compare shock normals, planarities, and Normal Incidence Frame cross-shock potentials determined from electric field measurements and proxies, for a subcritical interplanetary shock and a supercritical bow shock. The subcritical shock's cross-shock potential was 26 +/- 6 V. The shock scale was 33 km, too short to allow comparison with proxies from ion moments. Proxies from electron moments provided potential estimates of 40 +/- 5 V. Shock normals from magnetic field minimum variance analysis were nearly identical, indicating a planar front. The supercritical shock's cross-shock potential was estimated to be from 290 to 440 V from the different spacecraft measurements, with shock scale 120 km. Reflected ions contaminated the ion-based proxies upstream, whereas electron-based proxies yielded reasonable estimates of 250 +/- 50 V. Shock normals from electric field maximum variance analysis differed, indicating a rippled front.

Plain Language Summary An important problem in shock physics is understanding how the incoming plasma flow is thermalized across the shock. The role of the cross-shock electric field has not been well studied. We compare measurements and implicit estimates of cross-shock potential for a quasi-perpendicular weak (low Mach) shock and a quasi-perpendicular strong (moderate/high Mach) shock using data from the four Magnetospheric Multiscale satellites. The weak shock had lower cross-shock potential in the Normal Incidence Frame (about 30 V) than the strong shock (about 300 V). We also estimated the potential deduced from ion and electron data. Electron-based estimates agreed reasonably well with the measurements, but ion-based estimates encountered problems. The weak shock was too short compared to the ion data sampling period, while the strong shock reflected ions back into the upstream flow. Data from individual spacecraft indicated that the surface of the strong shock was not flat but rippled, one reason why its measured potential showed such a broad range.