Evidence for enhanced dynamic flow in ionospheric holes from the pioneer venus Orbiter Neutral Mass Spectrometer

Abstract

The Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) was operated in ion mode during the third nightside cycle of the Orbiter around Venus. Data have been acquired for the outbound ionospheric holes on orbit numbers 530 and 531. Holes are observed as deep troughs in the electron and ion density in the antisolar ionosphere of Venus, primarily between 23 and 2 hours local solar time, usually in north-south pairs and in conjunction with enhanced sunward or antisunward magnetic fields. Several models based on Orbiter data suggest that the holes are also regions of enhanced ion flow speeds. Spin modulated ion data acquired from the ONMS for species He +, C +, N + and O +, were reduced to obtain relative ion composition. The time of the maximum in the apparent ion flow observed in the spacecraft reference frame relative to that predicted by the spacecraft velocity is a measure of one component of the ion drift in the ecliptic plane. Although it is difficult to determine this component in the hole region due to the steep density gradients and low particle densities (< 10 cm −3), its minimum magnitude in the holes is of the order of 3 km s −1 for O +. This is larger than that observed in the ionosphere immediately surrounding the hole. The minimum ion drift speeds in the hole regions are largest for O + and smallest for He + with C + and N + intermediate. The concentrations of light ion species are enhanced relative to heavier species inside the hole as is evidenced by the increase in the He +/O + ratio. The measurements of ion drift in the holes support the suggestion that they are regions of enhanced dynamic flow. Dynamic flows have also been measured in orbit 500 which has no outbound hole. An increase in the minimum flow speed to about 1 km s −1 is observed as the ionopause is approached from the ionosphere side. The implication is that ions are being accelerated out of the ionosphere by the electric and magnetic fields present in the ionosheath region. The direction of the ion flow cannot be determined but it can be bounded in the ecliptic plane. Ion flow directions for O + inferred in this manner from the data are consistent with downward (toward Venus) flow within the outbound holes of orbits 530–531 and upward flow near the ionopause for orbit 500 without a hole. However, horizontal flow with a downward or upward component, respectively, cannot be excluded by the measurements.