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R.T. Clancy (Space Science Institute), B.J. Sandor (National Center for Atmospheric Research), G. Moriarty-Schieven (James Clerk Maxwell Telescope)
The mesospheric region of the Venus atmosphere lies above the massive sulfuric acid clouds, and plays critical roles in the photochemical stability and dynamical balance of the full Venus atmosphere. These include catalytic recombination of CO2, conversion of SO2 to sulfuric acid, and the transition from zonal rotation of the lower atmosphere to sub-solar to anti-solar flow within the upper atmosphere. Previous ground-based measurements of the upper portion (85-100 km) of this poorly observed region show global-scale diurnal variations in CO abundances, which are tied to the sub-solar to anti-solar circulation of the upper atmosphere; and long-term (>5-10 year) variations in atmospheric temperatures and night-side CO distributions, which are not understood. Diurnal variations within the lower mesosphere (60-85 km) have not been identified, yet global-scale variations in water and SO2 abundances have been observed with longer, if generally undefined timescales. We will present recent (1998-1999) millimeter and submillimeter observations of Venus mesospheric 12CO, 13CO, and HDO. These observations show a potential diurnal variation in lower mesospheric water vapor abundances and a dramatic cooling of the upper Venus mesosphere (down to 140 K at 90-100 km altitudes) which began between June and July of 1999.