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Abstract. 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 be 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 dosed 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.

Abstract.We study the plasma thermMization and acceleration process in collisionless magnetic reconnection by using a two-dimensional, particle-in-cell numericM simulation and discuss the plasma mixing process of cold lobe ions into the plasma sheet. We find that the plasma thermalization timescale is longer than the dynamic timescale of magnetic reconnection and that non-Maxwellian ion velocity distribution functions produced during the evolution of magnetic reconnection play an important role on the dynamical structure of the plasma sheet. The ion velocity distribution functions are characterized by four class of distributions: (1) anisotropic, high-speed ion beams along the magnetic field line in the plasma sheet boundary l•yer, (2) two counter-streaming ions along the magnetic field line inside the plasmoid and around the edge of magnetic diffusion region, (3) nongyrotropic, dumbbell-like ions near the X-type region, and (4) the thermal distribution function downstream of slow shocks. We compare the behavior of those ion velocity distribution functions obtained by our reconnection simulation with the distribution functions observed by the Geotail satellite. Most of ion dynamics observed in the Earth's magnetotail can be well understood by our kinetic reconnection simulation. We find that the plasma mixing between the meandering ions accelerated around the X-type region and the cold ions convected directly from the lobe without passing through the X-type region plays a significant role on the formation of non-M•xwelli•n ions.

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