Tailward electrons at the lobe-plasma sheet interface detected upon dipolarizations

We report on highly asymmetric spectra of electrons observed at the lobe-plasma sheet interface in the near-Earth (R < 15 R-E) magnetotail. The data were obtained as the Geotail spacecraft made the interface crossings when magnetic field dipolarizations were taking place. In the spectra, electrons in the 0.1 keV to a few keV energy range are seen to flow tailward, with their pitch angles highly collimated along the field lines. Energetic ion beams (> 10 keV) are occasionally detected flowing earthward counter to the above electrons. The large differences in the bulk flows result in an intense field-aligned current (FAC) downward into the ionosphere. The estimated vertical thickness of the FAC layer is found to be comparable to the relevant ion inertia length, leading us to propose that the tailward electrons sustain the downward FAC originating from the near-Earth X line driven by the Hall effects in magnetotail reconnection. Such a FAC structure is indeed shown by a three-dimensional Hall-MHD simulation to be located at the high-latitude edge of the plasma sheet. We also show the currents integrated over the Hall-FAC layer to be of significant amounts in the context of magnetosphere-ionosphere coupling.