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

J. A. Slavin, D. H. Fairfield, R. P. Lepping, M. Hesse, A. Ieda, E. Tanskanen, N. Østgaard, T. Mukai, T. Nagai, H. J. Singer, and P. R. Sutcliffe

Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771


Examination of observations taken by radially aligned ISTP spacecraft in the nightside magnetosphere on July 9, 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A 1-D 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 Re, respectively, ~2 to 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 Re. Dipolarization of the magnetic field at geosynchronous orbit, enhanced AKR emissions, Pi2 pulsations, high latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are 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, ~ 103 km/s. For both substorms, the magnetic field dipolarization at Geotail and GOES was found to be consistent with the magnetic flux pile-up scenario for dipolarization. In summary, our investigation strongly supports the modern 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, the generation of Pi2s, current wedge development and the other expansion phase phenomena, and, finally, ~10 to 20 min later, the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.

J. Geophys. Res., Vol. 107, No. A7, 10.1029/2000JA003501, 2002