The trapping and the thermalization of the outflowing ionospheric plasma in the outer plasmaspheric flux tubes during refilling after geomagnetic storms involves microscopic processes, which in the initial stage of the refilling are essentially collisionless. Although there are several observations on upflowing ions with anisotropic pitch angle distributions at superthermal energies in the region of the refilling, the role of such heated ions in the refilling has not been examined. It is shown here that an extended perpendicular ion heating by a low‐level background plasma noise near the ion cyclotron frequency with spectral power density ψ ∼ 10−11 V² m−2 Hz−1 can be effective in trapping the bulk of the ions in the flux tube. Such electric field noise levels can be generated by fluctuations associated with the equilibrium near marginal stability of a low‐density plasma mainly consisting of very hot ions originating from the ring current. The heated bulk ions have energies (∼ 1 eV) characteristic of the filled plasmasphere. When the turbulence level exceeds the above level, the heated ions show the features of the ion conies in the superthermal energy range observed along the field lines of refilling. In contrast to the weak extended heating, when the heating is intense and localized in the equatorial region, the trapped ions are shown to set up parallel electric fields pointing away from the equator. The potential drop associated with such fields are found to be large enough to stop interhemispheric flow by reflecting the ionospheric plasma streams.

Measurements of ion streams found to exist in the near wake of the shuttle Orbiter (Spacelab 2 mission) were used to examine the applicability of a one-dimensional, time-dependent plasma expansion model for wake filling. The results of the examinations show a good agreement between theory and experiment. This agreement implies that the measured streams can be interpreted in terms of collisionless plasma expansion.

During the Spacelab 2 mission, while the Plasma Diagnostics Package was attached to the Remote Manipulator System, differential ion vector measurements were obtained in the near wake at a distance of 4‐5 Shuttle radii. The Orbiter's wake was found to fill in at a much faster rate than can be explained by simple thermal motion. The measurements strongly suggest that filling of the Orbiter's wake is produced by the process of "collisionless plasma expansion into a vacuum" and that, for oblique angles of the magnetic field and velocity vectors, the near wake plasma depletion a few radii downstream is not sensitive to the body scale size.

Calculated results from a modified one-dimensional time-dependent plasma expansion model are compared with wake measurements o$ low energy H + ions made in the plasmasphere which consists of 77-90% H+ and 23-10% H. The variation of the theoretical-to-experimental results with the H+ ionic Mach numbe: (SH+) in the range 0.5 < SH+ < 1.2 is shown. It is found that the theoretical model used overestimates the particle flux in the wake by factors of 2 to 6. Possible causes for these differences are examined. Discussions regarding: (1) the shortcomings of using one-dimensional models and other simplifications commonly used in theoretical modeling, (2) the use of measurements from probes having finite angular acceptance in wake studies, and (3) a relative comparison between a three-dimensional neutral approximation and the modified one-dimensional model used here are given. This comparison shows that for the plasmaspheric parameters used here, the application of a neutral particle model yields reasonable results, since the ion thermal motion rather than the local electric field dominates the filling of the wake (particularly for weakly charged bodies). These discussions provide additional insight into such theory-experiment comparisons and should be of practical use.