LOCAL TIME ASYMMETRIES IN THE VENUS THERMOSPHERE

The bright emission at 130 nm from atomic oxygen in the Venus thermosp here, as measured by the Pioneer Venus Orbiter Ultraviolet Spectromete r (PVOUVS), can serve as a diagnostic of Venus thermospheric structure . Images from the PVOUVS at this wavelength reveal unusual large-scale patterns in the emission which can be interpreted as variations in th e O/CO2 ratio in the middle thermosphere. Through analysis of these im ages, a high-contrast asymmetry in local time has been revealed which has not previously been observed or predicted by any existing models. The asymmetry can be interpreted as typically a factor of 2 increase i n oxygen densities at the nighttime terminator over corresponding day times, and this basic pattern has been present in every image examined , spanning the period 1980-1990. The pattern in local time is not pres ent at equatorial latitudes less than approximately 30-degrees but app ears in both the northern and southern hemispheres poleward of 30-degr ees. This local time asymmetry in 0 can be understood in the context o f the subsolar-to-antisolar flow pattern which dominates the thermosph eric circulation: gravity waves, originating in the middle atmosphere of Venus where a westward zonal flow pattern dominates, will preferent ially enhance eddy mixing in the morning hours if these waves propagat e into the thermosphere. Enhanced eddy mixing tends to lower thermosph eric O. This same mechanism can also explain zonal asymmetries known t o be present in the thermospheric subsolar-to-antisolar wind pattern ( Alexander, 1992).