ON REMOVING MOLECULAR-IONS FROM VENUS

Acceleration or ''pickup'' of exospheric atomic oxygen ions by the int erplanetary convection electric field is a generally accepted mechanis m for the observed removal of O+ from Venus. However, heavier escaping molecular ions (e.g., O-2(+), CO2+, N-2(+), CO+, and NO+) in high abu ndances were also detected in the wake by the Pioneer Venus Orbiter (P VO) neutral mass spectrometer (ONMS) operating in its ion mode. It was recently demonstrated that pickup of O+ at low velocities from the te rminator upper ionosphere could explain some characteristics of the Ve nus ionospheric ''tail rays.'' Since the PVO ion mass spectrometer dat a indicate that a significant molecular ion component also contributes to the terminator ionosphere above the collisional region (greater th an or equal to 250 to 300 km altitude), we apply the tail ray model to study both the associated low-altitude O+ flows and the behavior of h eavier ions of similar origin. The predicted flow vectors show dawn/du sk asymmetries similar to those in the ONMS observations. Further, the heavier ions achieve higher peak energies, thus improving their chanc es of detection by the ONMS which has an energy threshold of similar t o 36 eV in the spacecraft frame. The appeal of this explanation is tha t no exotic or complicated interpretations are required, and that a br oad set of diverse observations fit a common scenario. The same mechan ism could in principle be operating at Mars where molecular ions were also detected in the wake on Phobos 2.