EVIDENCE FOR ION-TRANSPORT AND MOLECULAR ION DOMINANCE IN THE VENUS IONOTAIL

We present analyses from the five Pioneer Venus Orbiter plasma experim ents and the plasma wave experiment when a patch of plasma with enhanc ed densities was encountered in the near-Venus ionotail during atmosph eric entry at an altitude of similar to 1100 km in the nightside ionos phere. Our analyses of the thermal and superthermal ion measurements i n this plasma feature provides the first evidence that at times molecu lar ions in the 28-32 amu mass range are dominant over atomic mass spe cies thus yielding evidence for a transport mechanism that reaches int o the lower ionosphere. Analysis of plasma analyzer (OPA) observations at this time indicates the presence of ions measured in the rest fram e of the spacecraft at similar to 27 and 37 volt energy per unit charg e steps. In the rest frame of the planet these superthermal ions are f lowing from the dawn direction at speeds (assuming they are O-2(+)) of similar to 8 km/s and with a flow component downward (perpendicular t o the ecliptic plane) at speeds of similar to 2 km/s. OPA analyses als o determine the ion number flux, energy, flow angles, and angular dist ributions. Plasma wave bursts appear to indicate that plasma density d ecreases within and on the equatorward edge of the patch of enhanced p lasma densities are associated with ion acoustic waves and relative io n streaming.