Evidence for Ion Transport and Molecular Ion Dominance in the Venus Ionotail

We present analyses from the five Pioneer Venus Orbiter plasma experiments and the plasma wave experiment when a patch of plasma with enhanced densities was encountered in the near-Venus ionotail during atmospheric entry at an altitude of similar to 1100 km in the nightside ionosphere. Our analyses of the thermal and superthermal ion measurements in this plasma feature provides the first evidence that at times molecular ions in the 28-32 amu mass range are dominant over atomic mass species thus yielding evidence for a transport mechanism that reaches into the lower ionosphere. Analysis of plasma analyzer (OPA) observations at this time indicates the presence of ions measured in the rest frame of the spacecraft at similar to 27 and 37 volt energy per unit charge steps. In the rest frame of the planet these superthermal ions are flowing from the dawn direction at speeds (assuming they are O-2(+)) of similar to 8 km/s and with a flow component downward (perpendicular to the ecliptic plane) at speeds of similar to 2 km/s. OPA analyses also determine the ion number flux, energy, flow angles, and angular distributions. Plasma wave bursts appear to indicate that plasma density decreases within and on the equatorward edge of the patch of enhanced plasma densities are associated with ion acoustic waves and relative ion streaming.