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

We report on highly asymmetric spectra of electrons observed at the lobe-pl asma sheet interface in the near-Earth (R < 15 R-E) magnetotail. The data w ere obtained as the Geotail spacecraft made the interface crossings when ma gnetic field dipolarizations were taking place. In the spectra, electrons i n the 0.1 keV to a few keV energy range are seen to flow tailward, with the ir pitch angles highly collimated along the field lines. Energetic ion beam s (> 10 keV) are occasionally detected flowing earthward counter to the abo ve electrons. The large differences in the bulk flows result in an intense field-aligned current (FAC) downward into the ionosphere. The estimated ver tical thickness of the FAC layer is found to be comparable to the relevant ion inertia length, leading us to propose that the tailward electrons susta in the downward FAC originating from the near-Earth X line driven by the Ha ll effects in magnetotail reconnection. Such a FAC structure is indeed show n by a three-dimensional Hall-MHD simulation to be located at the high-lati tude edge of the plasma sheet. We also show the currents integrated over th e Hall-FAC layer to be of significant amounts in the context of magnetosphe re-ionosphere coupling.