MMS Reconnection Observations: Substorm Growth, Expansion, and Recovery Phases During 2017-2020 Tail Passes

The Magnetospheric Multiscale (MMS) four-spacecraft constellation has provided repeated opportunities to examine plasma, energetic particle, and magnetic field dipolarization behavior in the near-Earth tail region (r~20 RE) during reconnection events during magnetospheric substorms. We examine several isolated substorm sequences during 2017-2020. Upstream solar wind data show strong southward IMF Bz turnings that initiate substorm growth phase sequences. Such signatures are normally detected in the AE index and at geostationary orbit spacecraft (GOES, LANL). All four MMS spacecraft in the near tail region detect cases of strong tailward plasma flow, consistent with near-Earth X-line (NEXL) formation just Earthward of MMS. We thus infer that substorm onset in these cases was initiated by reconnection at X ~ -20 RE. Energetic electron and proton injections along with field dipolarization are also detected at GOES and Los Alamos spacecraft. The data suggests that the primary large-scale substorm plasmoid was rapidly ejected downtail at these times from the plasma sheet at expansion phase onset and this left the thin, residual plasma sheet tailward of r ~20 RE. This allowed the four closely spaced MMS spacecraft to all pass readily back and forth across the thin current sheet. MMS sensors often subsequently saw Earthward plasma flow implying that the NEXL had moved tailward of MMS. We interpret this as the signal of tailward retreat of the X-line during the substorm recovery time. MMS data indicate fast expansion of the plasma sheet thickness with strong energetic electron (E>50 keV) flux increase (including unidirectional particle streaming). Taken together, all these data show a repeatable sequence of expected substorm dynamical features of substorm growth, expansion and recovery phases clearly driven by powerful magnetic reconnection processes.