Local time asymmetries in the Venus thermosphere

The bright emission at 130 nm from atomic oxygen in the Venus thermosphere, as measured by the Pioneer Venus Orbiter Ultraviolet Spectrometer (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 the O/CO 2 ratio in the middle thermosphere. Through analysis of these images, 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 in 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 present at equatorial latitudes less than ∼30° but appears in both the northern and southern hemispheres poleward of 30°. This local time asymmetry in O can be understood in the context of the subsolar‐to‐antisolar flow pattern which dominates the thermospheric circulation: gravity waves, originating in the middle atmosphere of Venus where a westward zonal flow pattern dominates, will preferentially enhance eddy mixing in the morning hours if these waves propagate into the thermosphere. Enhanced eddy mixing tends to lower thermospheric O. This same mechanism can also explain zonal asymmetries known to be present in the thermospheric subsolar‐to‐antisolar wind pattern (Alexander, 1992).