The Pioneer Venus orbiter neutral mass spectrometer (ONMS) has made measurements of >36-eV ions in the altitude range 1300--3700 km for solar zenith angles greater than 120¿. The observations cover a time period from late 1982 to 1989. The superthermal ions form part of the near-tail region of Venus termed the ionotail. The ONMS superthermal ions represent only a small fraction (about 0.3%) of the plasma density in the ionotail region. The composition is mainly O+, but He+, N+, (CO++N+2), NO+, and O+2 have been identified. CO+2 is very rarely observed in this region, and H+ is not measured. The average O+ flux is about 105 cm-2 s-1, but higher fluxes from 106 to 108 cm-2 s-1 have been observed about 10% of the time. The directions of the apparent O+ ion flow in the ecliptic plane show predominantly tailward component with a smaller number of nontailward components. Since the energy of the superthermal ions is sufficient for planetary escape, the >36-eV O+ escape flux in the ionotail is estimated to be about 105 cm-2 s-1. Other species observed also have enough energy to escape. The O+ flux data show a factor of 2.5 increase from solar minimum to solar maximum implying a photoionization source for these ions. Neither the origin of the >36-eV ions nor the acceleration mechanism is precisely known. The O+ flux observations do not appear to be correlated with the direction of the ''cross-tail'' magnetic field as might be expected if the ions were due to the asymmetric pickup of newly ionized atmospheric neutrals above the ionopause.

The composition of the superthermal ions in the ionotail suggests that their source is most likely the high-altitude nightside ionosphere where O+ and not O+2 is the dominant ion. Transport of superthermal O+ across the terminator to the nightside has been observed, and measurements in the ionotail region at solar minimum near 2000 km show that O+ is mainly superthermal. Much further down the tail, very energetic (0--8 keV) O+ has been observed as it escapes Venus. Conditions in the lower nightside ionosphere have been shown to be consistent with an upward ion flow. Parallel electric fields or J¿B forces associated with the convection of the interplanetary magnetic field through or above the ionosphere have been suggested as acceleration mechanisms in the ionotail. Similar mechanisms have been proposed for the tail region of Mars where energetic molecular ions such as O+ have also been observed and may also be appropriate for Titan if the 28-amu species observed is due to H2CN+. ¿ American Geophysical Union 1991