Solitary electrostatic structures involving density depletions have been observed in the upper ionosphere by the Freja satellite [Dovner et al., 1994]. If these are interpreted as ion sound solitons, the difficulty arises that the standard Korteweg‐de Vries description predicts structures with enhanced rather than depleted density. Here we show that the presence of non‐thermal electrons may change the nature of ion sound solitary structures and allow the existence of structures very like those observed.

We show that a non-thermal electron distribution may change the nature of ion acoustic solitons. If the ions are assumed to respond as a fluid to perturbations in the potential, with no significant trapping in a potential well, then a thermal plasma only supports a solitary waves with a density peak. However, with a suitable distribution of non-thermal particles, solitary waves with both density peaks and density depressions may coexist. This may have applications to magnetosperic observations, where solitary structures with lowered densities have been observed in regions where the electron distribution is also seen to be non-thermal.

Characteristic features of the oxygen ion (O +) outflow in the nightside of Mars have been studied. The data set used includes all high telemetry O + energy spectra observations made by the ASPERA particle instrument onboard the Phobos 2 spacecraft. Oxygen outflow within the optical shadow of Mars was detected on all orbits, but no clear structure was found when the ions were organised according to the cross-flow component of the interplanetary magnetic field (IMF). Oxygen ions in the tail outside the shadow were, in contrast, detected only on a few orbits and were related to fluctuating magnetosheath magnetic and electric fields with non-Parker spiral IMF orientation. The analysis indicates that O + ions within the optical shadow from a "steady", ever persisting outflow feature. The high energy O + ions in the tail outside the optical shadow are in contrast possibly a manifestation of the dynamic nature of the Martian tail, although effects associated to an asymmetric tail can not be fully excluded.

In this paper a theoretical analysis for negatively biased single and double spherical probes in a plasma is presented. It is assumed that the motion of ions with respect to the probes is radial and that the plasma electron distribution function is Maxwellian. Photoemission from the probes and secondary electron emission are not taken into account. The analysis provides a tool which may be used to derive plasma parameters directly from experimentally obtained double probe characteristics. In addition the analysis is applied to the case of an isolated negatively charged dust grain in a plasma. The probe theory indicates that the charge on such a dust grain, in a hydrogen or argon plasma at floating potential, is approximately the same as the charge that the grain would have in a vacuum, for the case in which the grain's radius is less than or approximately equal to one tenth of the Debye length of the ambient plasma. At larger values of the grain radius the charge on the grain increases with the grain radius.