Spatial Heterodyne Imager for Mesospheric Radicals on STPSat-1

The Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER) was a high‐resolution, near ultraviolet spectrometer that imaged the Earth's limb for 2.5 years between March 2007 and October 2009. The instrument used the Spatial Heterodyne Spectroscopy technique for the first time on a satellite and successfully demonstrated its capabilities. SHIMMER measured the solar resonance fluorescence of the OH A2Σ+‐X2Π (0, 0) band around 309 nm, which allows the retrieval of mesospheric OH density profiles. It also measured the Rayleigh scattered background from the clear atmosphere and solar scattering from polar mesospheric cloud particles. We present details on the SHIMMER mission, the payload design, and the data analysis. A comparison between SHIMMER and concurrent Microwave Limb Sounder OH data shows good agreement between 60 and 90 km altitude for several latitudes and seasons. We also find good agreement of the SHIMMER OH densities and standard photochemical model calculations between 60 and 80 km. We find no evidence of a 25%–35% mesospheric OH deficit, previously reported using Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) OH data. However, independent analysis of Rayleigh scattered background signals observed by SHIMMER and MAHRSI under similar lighting conditions revealed that MAHRSI radiances are systematically smaller than SHIMMER radiances by 24%. Although this difference is well outside of the combined uncertainties for both experiments, the agreement of SHIMMER OH with Microwave Limb Sounder OH and standard photochemistry results, together with our Rayleigh scattering comparison, suggests an unidentified MAHRSI calibration problem that effectively eliminates the mesospheric OH deficit reported using MAHRSI observations.