In this review we call attention to basic phenomena and physical processes involved in the expansion of a plasma into a vacuum, or the expansion of a plasma into a more tenuous plasma, in particular the fact that upon the expansion, ions are accelerated and reach energies well above their thermal energy. Also, in the process of the expansion a rarefaction wave propagates into the ambient plasma, an ion front moves into the expansion volume, and discontinuities in plasma parameters occtlr. We discuss the physical processes which cause the above phenomena and point toward their possible application for the case of the distribution of ions and electrons (hence plasma potential and electric fields) in the wake region behind artificial and natural obstacles moving supersonically in a rarefied space plasma. To illustrate this, some in situ results are reexamined. Directions for future work in this area via the utilization of the Space Shuttle and laboratory work are also mentioned.1.

A study of electron temperature (7',) measurements made by a Langmuir probe mounted on the skin of the Explorer 31 satellite indicates that electron temperature in the very near wake exceeds that of the ambient electron gas. Potential causes of such an enhancement are mentioned but the possibility that the wake 'temperatures' reflect the existence of non-Maxwellian energy distributions cannot be discounted. The results are compared with observations from the Gemini/Agena wake experiment and are found to be in reasonable agreement.

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Measurements of ion current, electron temperature, and values of space potential obtained from the cylindrical electrostatic probe on board the Atmosphere Explorer C (AE-C) satellite were used to examine, in a parametric manner, the angular distribution of charge around the satellite. Interest is focused on nighttime equatorial data in the altitude range 275-620 km, which yields a wide range for the parameter RD (=Ro/)ko, where Ro is the radius of the satellite and 3,0 is the ambient value of the Debye lengths), including Ro > 10 •', which is of practical significance to large space platforms. The variations of normalized ion current in the wake zone of the AE-C satellite appear to display an exponential dependence on R o for 'constant' values of other relevant parameters. The angular variations of electron temperature (Te) and space potential (4s) in the close vicinity of the satellite's surface were examined and compared with results from the Explorer 31 satellite. The variation of the ratio 3• = ½s(measured)/ ½•(computed) with Te was examined using data from both the AE-C and the Explorer 31 satellites. Itwas found that 3• > 1. Possible causes for the above inequality are discussed. A. 513 mainly to the well-known difficulties of achieving a self-consistent solution of the Vlasov-Poisson equations for practical boundary conditions and plasma parameters. The available theories differ mathematically in the numerical approaches adopted in order to obtain approximate solutions. More important, the theories often employ physical assumptions whose validity and range of applicability to satellite-ionosphere interaction are subject to controversy. A possible way of gaining a good physical insight into the degree and range for the validity and applicability of the assumptions used in the theoretical models is via theory-experiment comparisons. Such comparisons should be performed using in situ measurements conducted for the widest possible range of plasma parameters. Unfortunately, no such studies have been performed in an extensive and physically meaningful manner, except for the studies of Gurevich et al. [1970], Sarnir and Jew [1972], Gurevich and Dimant [ 1975], and Sarnir et al. [ 1975], which used a relatively small sample of in situ measurements under a restricted range of plasma parameters. A major reason for the latter shortcoming is the meager and limited amount of in situ measurements available at the present time for such studies. More detailed discussions on the state of knowledge are given elsewhere [e.g., Samir, 1973; Samir et al., 1977; Fournier and Pigache, 1975; Gurevich and Pitaevskii, 1975; Liu, 1975; Al'pert, 1976].The wide complement of instruments on the Atmosphere Explorer satellites described in great detail elsewhere (see the special edition of Radio Science on the Atmosphere Explorer satellites, 8(4), 1973) provides an opportunity to study the angular distribution of charged particles around the satellites, where all the relevant plasma parameters are known. Such an opportunity did not always...