An Investigation of the Ozone Balance in the Upper Mesosphere and Lower Thermosphere Using MSX/UVISI Stellar Occultation and Airglow Measurements

Although the bulk of terrestrial ozone exists in the stratosphere, ozone is one of the most important species in the upper mesosphere and lower thermosphere (UMLT) owing to its role in the UMLT energy balance. Absorption of solar UV radiation by ozone is the primary heating source in the UMLT region. Consequently, the spatial and temporal distribution of ozone affects the latitudinal and seasonal variability of the mesospheric thermal structure. To date, the UMLT ozone balance has not been rigorously evaluated owing to a lack of the necessary simultaneous measurements. For some time, measurements of [O3] in the UMLT region obtained by various methods (mass spectrometry, emission spectroscopy, and stellar occultation) have often exceeded model-predicted values. This discrepancy suggests that either our basic understanding of UMLT ozone chemistry is incomplete or that the atmospheric states (thermal, dynamical, and [O] and [H] composition) predicted by the models are incorrect. This problem is therefore of fundamental importance to our understanding of the processes responsible for the global structure of the UMLT region and needs to be resolved. We are using the unique capabilities of the Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI) on the Midcourse Space Experiment (MSX) satellite to investigate the UMLT ozone balance. From MSX/UVISI stellar occultation measurements, we can determine the ozone and molecular oxygen density profiles in the UMLT region, while simultaneous measurements of the UMLT airglow emissions provide a means to infer [O], [H], and temperature. Through an analysis of these simultaneous measurements, we will be able to determine whether or not the current nighttime UMLT ozone Ox-HOx chemical scheme is complete and to pinpoint the areas in which current atmospheric models may be deficient. We will report on the results of our investigation.