High-Resolution Measurements of the Cross-Shock Potential, Ion Reflection, and Electron Heating at an Interplanetary Shock by MMS

The Magnetospheric Multiscale (MMS) spacecraft obtained unprecedented high-time resolution multipoint particle and field measurements of an interplanetary shock event on 8 January 2018. The spacecraft encountered the supercritical forward shock of a forward/reverse shock pair in the pristine solar wind upstream of the bow shock near the subsolar point as they neared apogee at ~25 R_E. The high-time resolution measurements from the four spacecraft, separated by only ~20 km, allowed direct measurement of particle distributions revealing evidence of electron heating and near specularly reflected ions. The cross-shock potential is calculated directly from 3-D electric field measurements. This is the first reported direct high temporal resolution (<1 s) observation at an interplanetary shock of near specularly reflected ions. Calculation of the cross-shock potential yields a potential jump significant enough to reflect at least some of the protons from the incident solar wind beam. The cross-shock potential calculated here is consistent with previous estimations based on particle measurements and numerical/analytical simulations. The ambipolar contribution to the cross-shock potential calculated from the four-spacecraft divergence of the electron pressure tensor is somewhat higher than that inferred form the Liouville-mapped electron energy gain across the shock. Furthermore, the high-time-resolution 3-D electric field measurements reported here reveal small-scale nonlinear structures embedded in the shock layer that contribute to the nonmonotonic shock transition.