Plasma distribution functions in plasmoids: GEOTAIL observations Original Research Article

GEOTAIL observations have revealed that ion velocity distribution functions in plasmoids are in general non-Maxwellian, and often contain beams, which represent the memory of their source and route associated with magnetic reconnection processes. Cold ions of the lobe origin flow into the plasmoid along magnetic field lines, are accelerated and heated through slow shocks at the boundary, and finally merge with hot components inside the plasmoid. Deep inside the plasmoid, however, their remnants are still discernible and the ion distribution functions often show the existence of counterstreaming ion beams. In this region both ends of the field lines are most likely connected to IMF through the northern and southern lobes with short distances, so that ions of the lobe origin have not had time enough for thermalization. The presence of a long-duration post-plasmoid plasma sheet (PPPS) with high tailward speeds shows the continuing occurrence of reconnections on the earthward side of the spacecraft location. The ion velocity distributions contain beams almost everywhere in the PPPS region, but their appearance is highly variable and at times non-gyrotropic. In the near-Earth region, plasma flow properties are also highly variable and bursty after the plasmoid traversal. The high variability of plasma distribution functions and flow properties may represent a consequence of multiple and bursty reconnections, and coalescence of a number of vortices may constitute the structure of the post-plasmoid plasma sheet.