Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm44b..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SM44B-02
Other
2723 Magnetic Reconnection (7526, 7835), 2744 Magnetotail, 2790 Substorms, 7526 Magnetic Reconnection (2723, 7835), 7835 Magnetic Reconnection (2723, 7526)
Scientific paper
Time development of near-Earth magnetotail during substorms has been investigated by superposed-epoch analysis with Geotail data. The start time of each substorm was determined by auroral data obtained by Polar and Image spacecrafts. Key parameters derived from plasma and magnetic field data of Geotail were sorted in the X(GSM) - Z(estimated) coordinates, according to the method described in a paper by Machida et al. [JGR, 25291, 2000], in which we assume that an event with higher plasma-beta should be located closer to the plasma-sheet center. The result shows that the earthward flows exist in the plasma-sheet boundary layer of -20Re > X > - 30Re prior to the substorm onset. This flow is a foundation of the boundary layer model of substorm. The Poynting flux toward the plasma sheet center also enhances in the lobe region of -10Re > X > -15Re. This Poynting flux is a necessary condition for the thermal catastrophe model. Interestingly, earthward flow in the central plasma sheet of -13Re > X > -20Re starts about 4 min before the onset. After t = 0, i.e., the auroral break-up, the dipolarization and the tailward flows with southward magnetic fields of plasmoid start. Those variations are predicted by the current disruption model, the ballooning instability model, and the near-Earth neutral line model. The initial location of the total pressure decrease is obscured but it seems to start in the central plasma sheet of -10Re > X > -20Re at t ~ -4 min. And, this variation propagates to the surrounding regions successively. We could confirm various variations which relevant models of substorm are based on or predict. However, non of them can perfectly explain our results. Thereby, we propose a new model, in which an earthward convective flow starts first about 4 min before the onset, and this earthward flow induces the ballooning instability or other instability to cause the current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, namely, the relaxation of a highly stretched sling-shot current sheet produces the enhanced duskward electric field at its tailward edge, which is the boundary between the sling-shot current with large stress and the Harris-type current sheet with little stress. Further, induced flows toward the current sheet center around the boundary may enhance the formation of the magnetic neutral line and the efficiency of the magnetic reconnection.
Ieda A.
Machida Shiki
Miyashita Yuki
Mukai Tadashi
Nagai Takaya
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