NEUTRAL COMPOSITION MEASUREMENTS BY THE PIONEER VENUS NEUTRAL MASS-SPECTROMETER DURING ORBITER REENTRY

Measurements by the Pioneer Venus Orbiter Neutral Mass Spectrometer (O NMS) instrument during Orbiter re-entry have been made from 18-24 hour s local solar time, above 170 km, of He, and from midnight to 4.5 hour s below 200 lan of He, N, O, CO, N2, and CO2. Preliminary results indi cate 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 s pecies 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 hi gher 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 sim ilar to that observed in 1978-80 confirming that thermosphere superrot ation is still present. Comparison with the results of a Venus Thermos phere General Circulation Model suggests the nightside is not sensitiv e to changes in solar activity due to the isolation of the day and nig ht thermospheres. Apparently, the relatively small changes in die days ide thermosphere with solar activity have little impact on the nightsi de thermosphere.