VENUS O2 VISIBLE AND IR NIGHTGLOW - IMPLICATIONS FOR LOWER THERMOSPHERE DYNAMICS AND CHEMISTRY

The National Center tor Atmospheric Research thermospheric general cir culation model for the Venus thermosphere is modified to examine two o bserved night airglow features, both of which serve as sensitive trace rs of the thermospheric circulation. New O2 nightglow data from the Pi oneer Venus Orbiter (PVO) star tracker (O2 Herzberg II at 400-800 nm) and ground-based telescopes (O2 IR at 1.27 mum) (Allen et al., 1992) y ield additional model constraints for estimating Venus winds over 100- 130 km. Atomic oxygen, produced by dayside CO2. photolysis peaking nea r 110 km, and transported to the nightside by the global wind system, is partially destroyed through three-body recombination. yielding the O2 Herzberg II visible nightglow. This emission is very sensitive to h orizontal winds at altitudes between 100 and 130 km. Other trace speci es catalytic reactions also contribute to the production of the very s trong nightside infrared (1.27 mum) emission. This paper examines the dynamical and chemical implications of these new data using the Venus thermospheric general circulation model (VTGCM) as an analysis tool. T hree-dimensional calculations are presented for both solar maximum and solar medium conditions, corresponding to early PVO (1979-1981) and P VO entry (mid-1999) time periods. Very distinct periods are identified in which zonal winds are alternately weak and strong in the Venus low er thermosphere. VTGCM sensitivity studies are conducted to assess the impacts of potential changes in thermospheric zonal and day-to-night winds, and eddy diffusion on the corresponding nightglow intensities. It appears that cyclostrophic balance extends above 80 km periodically , owing to a reversal of the upper mesosphere latitudinal temperature gradient, and thereby producing strong zonal winds and correspondingly modified O2 nightglow distributions that are observed.