Simultaneous observations of earthward flow bursts and plasmoid ejection during magnetospheric substorms

Examination of observations taken by radially aligned International Solar Terrestrial Physics spacecraft in the nightside magnetosphere on 9 July 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A one‐dimensional model of plasma sheet flow is applied to these observations to determine the time and location for the initiation of lobe flux tube reconnection. For the single clear flow burst‐plasmoid pair observed during the first substorm and the three pairs produced by the second substorm, lobe flux reconnection was inferred to have started at X ∼ −15 to −18 R E , respectively, about 2–5 min prior to the observations of substorm expansion phase onset. These time delays and propagation speeds are shown to be consistent with the measured plasma sheet bulk flow speeds. Substorm expansion phase onset was essentially coincident with the arrival of the flow bursts at Geotail, which was located near the inner edge of the plasma sheet at X ∼ −9 R E . The dipolarization of the magnetic field at geosynchronous orbit, auroral kilometric radiation (AKR) emissions, Pi2 pulsations, high‐latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are all coincident within the ±1 min resolution of the measurements. Accordingly, it appears that earthward of the inner edge of the plasma sheet, where Geotail was located, substorm effects propagated at speeds comparable to the Alfven speed characteristic of the high‐latitude inner magnetosphere, ∼10 3 km s −1 . In summary, the results of our investigation strongly support the modern near‐Earth neutral line (NENL) model of substorms in which the onset of lobe flux tube reconnection in the near tail is followed ∼2–5 min later by the braking of earthward flow bursts as they encounter the inner magnetosphere and within ∼1 min, by Pi2s generations, current wedge development, and AKR and auroral expansion, and finally, ∼10–20 min later, by the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.