On removing molecular ions from Venus

Abstract

Acceleration or “pickup” of exospheric atomic oxygen ions by the interplanetary convection electric field is a generally accepted mechanism for the observed removal of O+ from Venus. However, heavier escaping molecular ions (e.g., O2+, CO2+, N2+, CO+, and NO+) in high abundances were also detected in the wake by the Pioneer Venus Orbiter (PVO) neutral mass spectrometer (ONMS) operating in its ion mode. It was recently demonstrated that pickup of O+ at low velocities from the terminator upper ionosphere could explain some characteristics of the Venus ionospheric “tail rays.” Since the PVO ion mass spectrometer data indicate that a significant molecular ion component also contributes to the terminator ionosphere above the collisional region (≥250 to 300 km altitude), we apply the tail ray model to study both the associated low‐altitude O+ flows and the behavior of heavier ions of similar origin. The predicted flow vectors show dawn/dusk asymmetries similar to those in the ONMS observations. Further, the heavier ions achieve higher peak energies, thus improving their chances of detection by the ONMS which has an energy threshold of ∼36 eV in the spacecraft frame. The appeal of this explanation is that no exotic or complicated interpretations are required, and that a broad set of diverse observations fit a common scenario. The same mechanism could in principle be operating at Mars where molecular ions were also detected in the wake on Phobos 2.