The transport of plasma sheet material from the distant tail to geosynchronous orbit

Several aspects of mass transport in the Earth's plasma sheet are examined. The evolution of plasma sheet material as it moves earthward is examined by statistically comparing plasma sheet properties at three different downtail distances: near-Earth plasma sheet properties obtained from measurements by 1989-046 near the geomagnetic equator near midnight at 6.6 R-E, midtail plasma sheet properties obtained from ISEE 2 measurements during 333 encounters With the neutral sheet, and distant-plasma sheet properties obtained from ISEE 2 measurements during 53 encounters with the interface between the plasma sheet and the plasma sheet boundary layer. Examination of the evolution of the plasma sheet through pressure-density space shows that the transport is nearly adiabatic (gamma = 1.52), with a loss of entropy observed in the near-Earth region. The estimated pressure loss from the plasma sheet associated with the aurora is able to account for the observed decrease in entropy. The near-Earth plasma sheet plasma is also found to be compressed much less than would be expected from magnetic field models. Examination of the evolution of the plasma sheet through density-flux tube-volume space (with the aid of the T89c magnetic field model) indicates that there is a substantial loss of mass from plasma sheet flux tubes. Global magnetic reconnection during substorms and, patchy reconnection at other times is invoked to account (1) for the required mass loss, (2) for the related lack of compression, and (3) for an observed disconnection between ionospheric convection and plasma sheet convection. This reconnection must occur closer than 20 R-E downtail. Selective transport is examined by statistically analyzing the ISEE 2 neutral sheet crossing data set: strong transport is found to be associated with low densities, with weak B-z, and with large flux tube-volume. A correlation between the direction of the flow in the plasma sheet and the solar wind velocity indicates that earthward transport is stronger when the solar wind velocity is lower. An examination of near-Earth and of midtail plasma sheet densities, temperatures, and entropies shows that the plasma sheet is usually spatially homogeneous, contrary to a "bubbles and blobs" picture of transport. Several new points of view about plasma sheet transport are discussed, including the dominant role of near-Earth reconnection, the importance of auroral zone pressure loss, the control of the plasma sheet properties by the density and speed of the solar wind, and the disconnection of the ionospheric and plasma sheet flow patterns.