Satellite measurements of hydroxyl in the mesosphere

The global distribution of hydroxyl (OH) in the middle atmosphere was recently measured by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) on a satellite deployed and retrieved by the space shuttle. During 75 orbits, MAHRSI acquired 1800 daytime limb scans of the OH ultraviolet solar resonance fluorescence intensity. Each limb scan extends over the altitude region from 30 to 90 km and across 10 degrees of latitude between 53 degrees S and 63 degrees N. OH number densities were retrieved using a Twomey regularization scheme constrained by the smoothness of the retrieved profile. Results provide a detailed description of the diurnal variation of mesospheric OH. Midmorning OH densities had a well defined peak of about 6 x 10(6) cm(-3) near 70 km, a broad minimum centered near 64 km, and rose to about 1 x 10(7) cm(-3) at 50 km. This profile is in substantial disagreement with photochemical model predictions [Summers et al., this issue]. The observations are compared with the two previous measurements.