Neutral composition measurements by the Pioneer Venus Neutral Mass Spectrometer during Orbiter re‐entry

Abstract

Measurements by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) instrument during Orbiter re‐entry have been made from 18–24 hours local solar time, above 170 km, of He, and from midnight to 4.5 hours below 200 km of He, N, O, CO, N2, and CO2. Preliminary results indicate that in the post‐midnight sector He is the dominant species above 170 km, O the dominant species from 140–170 km and CO2 the dominant species below 140 km. Estimated scale height temperatures for He, O and CO2 of about 105–120 K are similar to those observed in 1978–80 at higher solar activity. The densities at 1 am local solar time and at 150 km are within 35% of those measured earlier. The He bulge is also similar to that observed in 1978–80 confirming that thermosphere superrotation is still present. Comparison with the results of a Venus Thermosphere General Circulation Model suggests the nightside is not sensitive to changes in solar activity due to the isolation of the day and night thermospheres. Apparently, the relatively small changes in the dayside thermosphere with solar activity have little impact on the nightside thermosphere.