ALBERT

Description: Millimeter-wave observations of CO lines have provided a detailed picture of Venus' mesosphere dynamics in 1991 from simultaneous measurements of absolute wind velocities in two layers and of temperature and CO horizontal and vertical profiles at 75-115 km. Venus' circulation at 90-110 km was characterized in 1991 by the superposition of a zonal retrograde flow and a subsolar-to-antisolar flow of approximately equal velocities, increasing from about 40 +/- 15 m/sec at 95 km to 90 +/- 15 m/sec at 105 km altitude. The magnitude of the increase of the SS-AS flow is consistent with Venus thermosphere general circulation models (VTGCM). At 105 km, the data further indicate a cos(latitude) dependence of the zonal flow and marginally suggest the presence of a poleward meridional component of 35 +/- 30 m/sec. No obvious day-to-day variations of the circulation are evident in the data at the 20 m/sec level. Thermal profiles in the low-latitude region appear to be consistent with the Pioneer Venus nightside profile, except above 110 km, where they are somewhat colder. High-latitude warming is still found, but mid-latitudes appear to be colder than the equator. The atmosphere appears to be in cyclostrophic balance up to about 105 km. The horizontal distribution of CO on Venus' nightside is essentially uniform, both in latitude and in local time. This behavior agrees with VTGCM simulations in which the zonal flow velocity is prescribed to match the observations. Comparison with previous wind measurements indicates that the zonal flow experiences dramatic long-term variations. This variability, along with short-term fluctuations of the mesospheric zonal flow (evidenced by the variability in the O2 nightglow emissions), apparently controls the CO and O2 nightglow distributions. Gravity wave activity is a plausible mechanism that can drive these variations.