HR: 1340h
AN: SM13C-1464    [Abstracts]
TI: Partial Dropouts of the Near-Earth Plasma Sheet at Substorm Onsets
AU: * Vigilant, T C
EM: tvigilant@pvamu.edu
AF: Solar Observatory, Prairie View A&M University, MS 2250, PO Box 519, Prairie View, TX 77446-0519, United States
AU: Erickson, G M
EM: gmerickson@pvamu.edu
AF: Solar Observatory, Prairie View A&M University, MS 2250, PO Box 519, Prairie View, TX 77446-0519, United States
AB: This study utilizes particle and field data observed at the Geotail satellite in the nightside plasma sheet at -9 R_E > X_GSE > -15 R_E to describe the partial dropout of the plasma sheet at times of substorm onsets. At distances -15 R_E > X_GSE > -24 R_E, dropout of the plasma sheet and entry of an observing satellite into the tail lobe is the dominant signature of substorm onset. The plasma sheet expands back over the satellite near the beginning of substorm recovery [ Hones et al., J. Geophys. Res., 75, 7060, 1970]. For X_GSE < -24 R_E the dominant signature is the plasmoid [e.g., Baumjohann et al., J. Geophys. Res., 104, 24,995, 1999]. These signatures are understood within the context of the near-Earth X-line model. At locations X_GSE > -15 R_E dipolarization and earthward plasma injection are the usual features at onset. Near the center of the plasma sheet (|B_x/B_z| < 2.5), dipolarization/injection is nearly always coincident with onset. However, in a significant number of onsets, when |B_x/B_z| > 2.5, injection is delayed, and instead, a partial dropout of the plasma sheet commences at onset. The partial dropout as described here is reminiscent of a plasma-sheet dropout, but the satellite does not enter the tail lobes. One type of partial dropout that we will describe here has the following consistent set of characteristics: (1) it begins coincident with the first signs of ground Pi2 onset; (2) the density, temperature and plasma beta decrease toward lobe values; (3) B_z increases approximately linearly while B_x remains approximately constant; (4) |V_z| increases consistent with motion of plasma and magnetic flux toward the center of the plasma sheet; and (5) at the satellite's location, plasma-sheet recovery is coincident with plasma injection and dipolarization. While the partial dropout is not an unusual signature at onset, neither is it unusual for this signature to be absent. Our interpretation is that the satellite is located in a region of transition between the dipolar, inner plasma sheet and the thin, expansion-phase, middle plasma sheet. We suspect that the satellite's location maps tailward along the magnetic field to the equatorial region of the dropped-out, thin plasma sheet.
DE: 2740 Magnetospheric configuration and dynamics
DE: 2760 Plasma convection (2463)
DE: 2764 Plasma sheet
DE: 2790 Substorms
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting