Evidence for a Solar Cycle Variation in Nightglow and Atomic Oxygen in the Mesosphere and Lower Thermosphere.

Six years of UARS/WINDII nightglow data have been used to derive atomic oxygen mixing ratios from the 80 to 110 km. This data set extends from winter 1991 to 1996 and ranges from shortly after solar maximum to solar minimum. Inter-annual variations in the derived mixing ratio are observed. This variation is in addition to the local time, seasonal and annual variations previously identified in this data set. Above ~90 km there is a positive correlation between the solar cycle and the oxygen mixing ratio as would be expected given the solar cycle dependence of its source in the thermosphere (dissociation of molecular oxygen). The decrease at these heights is by more than 50%. The correlation changes sign below this height and the mixing ratio increases by about 50%. A decrease in the peak height of the hydroxyl and oxygen green line nightglow layer is also observed during this time period. Associated variations are expected in the chemistry and energetics of the mesosphere and lower thermosphere.