Substorm Triggering by New Plasma Intrusion 2: Incoherent-Scatter Radar Observations

A critical and long-standing problem for physical understanding of substorms is determination of the sequence of events that lead to onset. In a companion study, we have identified a remarkably repeatable sequence of events in observations from the multi-station array of THEMIS all-sky imagers (ASI). Here we use incoherent scatter radar (ISR) observations to obtain evidence supporting the proposal for the pre-onset timing sequence put forward in the companion study based on the ASI observations. We suggested that first new plasma crosses the polar-cap boundary and enters the plasma sheet with lower global entropy than that of the pre-existing plasma sheet plasma. This new lower entropy plasma then moves equatorward as a flow channel (earthward in the equatorial plane) bringing the new plasma to the onset region. This intrusion of new plasma leads to the substorm onset instability in the near-Earth plasma sheet. We suggested that new plasma entering the plasma sheet into the dusk convection cell moves equatorward around the Harang flow shear and leads to pre-midnight onset. New plasma entering into the dawn cell drifts equatorward and eastward, resulting in near- and post-midnight onset. Using the Sondrestrom ISR, we find that enhanced flow commonly enters the plasma sheet from the polar cap ~ 8 min prior to onset, and is related to PBI-related density enhancements, consistent with inferences from the ASI observations. Using the Poker Flat ISR (PFISR), we find that evidence that, within a few min of onset, an enhanced westward flow reaches the SAPS region equatorward of the Harang reversal (dusk-cell onsets) or enhanced eastward flow enters the onset region (dawn-cell onset), which is also as inferred from the ASI observations. PSISR observations of E-region ionization due to proton precipitation indicate that the enhanced flows indeed do consist of reduced entropy plasma, and that onset occurs poleward of much of the enhanced SAPS flow (dusk-cell onsets) and equatorward of the eastward flows (dawn-cell onsets)