Venus O2 visible and IR nightglow: Implications for lower thermosphere dynamics and chemistry

Abstract

The National Center for Atmospheric Research thermospheric general circulation model for the Venus thermosphere is modified to examine two observed night airglow features, both of which serve as sensitive tracers of the thermospheric circulation. New O2 nightglow data from the Pioneer Venus Orbiter (PVO) star tracker (O2 Herzberg II at 400–800 nm) and ground‐based telescopes (O2 IR at 1.27 μm) (Allen et al., 1992) yield additional model constraints for estimating Venus winds over 100–130 km. Atomic oxygen, produced by dayside CO2 photolysis peaking near 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 horizontal winds at altitudes between 100 and 130 km. Other trace species catalytic reactions also contribute to the production of the very strong nightside infrared (1.27 μm) 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. Three‐dimensional calculations are presented for both solar maximum and solar medium conditions, corresponding to early PVO (1979–1981) and PVO entry (mid‐1992) time periods. Very distinct periods are identified in which zonal winds are alternately weak and strong in the Venus lower 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.