Plasma entry from the flanks of the near-Earth magnetotail: Geotail observations

Geotail observations of the low-latitude boundary layer (LLBL) in the near-Earth tail flanks are reported. Cold-dense stagnant ions, which are likely to he of the magnetosheath origin, are detected in this region of the magnetosphere. Charge neutrality is maintained by accompanying dense thermal (< 300 eV) electrons presumably also from the magnetosheath. Compared to the magnetosheath component, however, the electrons are anisotropically heated to have enhanced bidirectional flux along the field lines. The enhanced bidirectional flux is well balanced, and this fact, together with the slow convection, suggest the closed topology of the field lines. In addition to these common characteristics, a dawn-dusk asymmetry is observed in data for several keV ions, which is attributed to the dawn-to-dusk cross tail magnetic drift of the plasma sheet ions. We also show a case that strongly suggests that this entry of cold-dense plasma from the magnetosheath via near-Earth tail flanks can be significant at times. In this case, the cold-dense plasma is continuously detected as the spacecraft moves inward from the magnetospheric boundary to deep inside the magnetotail. By referring to the solar wind data showing little dynamic pressure variation during the interval, we interpret the long duration of the cold-dense status as indicative of a large spatial extent of the region: The cold-dense plasma is not spatially restricted to a thin layer attached to the magnetopause (LLBL) but constitutes an entity occupying a substantial part of the magnetotail, which we term as the cold-dense plasma sheet. The continuity of the cold-dense plasma all the way from the boundary region supports the idea that the magnetosheath plasma is directly supplied into the cold-dense plasma sheet through the flank.