Plasma entry across the distant tail magnetopause 1. Global properties and IMF dependence

[1] As many as 1800 magnetopause crossings were identified utilizing the plasma and magnetic field data obtained by the GEOTAIL spacecraft in the distant (−100 RE > X > −210 RE) tail region. We classified the magnetopause crossings into three (open, closed, and ambiguous) types based on the jump conditions for the MHD discontinuities and statistically investigated the properties of these boundaries. The most conspicuous finding is that for a considerable percentage (∼25%) of the lobe-magnetosheath crossings, the magnetotail boundary is locally open for northward as well as southward IMF polarities, indicating that magnetic reconnection frequently occurs regardless of the polarity of the IMF Bz. Our statistics confirm that the magnetic field has a finite normal component for the locally open boundaries but a negligibly small one for the closed boundaries. The field lines threading the open boundary are directed inward (outward) in the Northern (Southern) Hemisphere, supporting the view that such field lines are connected to the Earth. The jump in the plasma flow velocity at the open boundaries agrees well in magnitude and direction with the ones expected for the rotational discontinuity. The flow direction is directed into the magnetotail, which is again consistent with the model of the rotational discontinuity. The spatial distribution of the open boundaries suggests that the locally open portions of the magnetotail boundary migrate toward high latitudes for southward IMF cases and toward low latitudes (toward the plasma sheet) for northward IMF cases. These results indicate that the solar wind plasma is effectively transferred to the lobe/mantle region through the open portion of the boundary whose location is controlled by the direction of the IMF.