Inter-seasonal observations of Mesospheric Hydroxyl by SHIMMER (Invited)

The Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER) on STPSat-1 is a high resolution, UV, mesospheric, limb sounder that uses the innovative optical technique of Spatial Heterodyne Spectroscopy (SHS). For extended sources, SHS facilitates significant size and weight savings when compared to conventional grating spectrographs with similar spectral resolution and sensitivity. SHIMMER images the limb near 309nm with a spectral resolution of ~250mÅ, which allows for the observation of the OH A2Σ+--X2Π(0,0) band. From these observations we retrieve OH density profiles from 90 to 60 km altitude. SHIMMER data covers more than 2.5 years starting in April 2007. The low inclination orbit and seasonal yaw cycle allow SHIMMER to measure up to about 58° latitude in the summer hemisphere and up to about 12° in the winter hemisphere. The orbit precesses approximately 30 minutes per day, allowing the investigation of diurnal variations. In this presentation we compare SHIMMER OH profiles with coincident Microwave Limb Sounder (MLS) data from the Aura mission and photochemical model calculations for different latitudes and seasons. We will also compare the SHIMMER OH results with measurements taken by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) in 1997. In particular, we will revisit the previously reported mesospheric OH model excess and what impact the new SHIMMER data-set has on this problem.