Optical Spectrograph and Infra-Red Imaging System (OSIRIS) observations of mesospheric OH A~2Σ~+-X~2Π 0-0 and 1-1 band resonance emissions

摘要

Although only a minor species, the OH molecule plays an important role in the photochemical control of mesospheric ozone density and has been the target of a number of observational programs, principally through the OH A~2Σ~+-X~2Π 0-0 band emission at 308 nm. This emission band arises from solar resonance fluorescence excitation of OH X~2Π ground state molecules, and its observation is complicated by the presence of an underlying atmospheric Rayleigh scattering spectrum. We show that the OH A~2Σ~+-X2~Π 0-0 band emission has been reliably and routinely detected with the moderately low, 0.9 nm, spectral resolution Optical Spectrograph and Infra-Red Imaging System (OSIRIS) limb scanning spectrograph. Changes in upper mesospheric water vapor observed by the Halogen Occultation Experiment (HALOE) are readily detected as changes in the OH density profiles seen by OSIRIS. Altitude profiles of OH density in the middle and upper mesosphere are in good agreement with model results that incorporate coordinated HALOE water vapor measurements. The agreement is within the HALOE and OSIRIS error limits when the recommended standard reaction rates are assumed. Conversely, model calculations of OH density using the revised reaction rates proposed to explain the Middle Atmosphere High-Resolution Spectrograph Investigation (MAHRSI) OH profiles typically fall outside the observed OSIRIS error limits. The OSIRIS results suggest that the probable difference between the observed and modeled OH densities is less than 15% from 55 to 80 km. Diurnal and seasonal OH variations observed by OSIRIS are in good agreement with model predictions. These successful comparisons suggest that the OSIRIS OH database, spanning more than 4 years of operation and broad ranges of latitude and local time, can contribute significantly to studies of OH photochemistry and upper mesospheric water vapor.