OSIRIS observations of OH A2Σ+-X2Π 308 nm solar resonance fluorescence at sunrise in the upper mesosphere

Since the OH molecule plays a critical role as a catalyst in atmospheric photochemistry, an accurate measurement of the OH density profile covering a broad range of latitudes and solar local times is required to quantify the major reactions involved. The optical spectrograph and infra-red imager system (OSIRIS) instrument on the Odin satellite observes scattered solar radiation at the terrestrial limb from the upper troposphere, the stratosphere, and the mesosphere. The wavelength range, 275 nm to 810 nm includes the OH A(2)Sigma(+)-X-2 Pi 0-0 band at 308 nm, which is seen in solar resonance emission superimposed upon the underlying atmospheric Rayleigh-scattering background. OSIRIS routinely detects the OH 308 nm emission in the mesosphere from sunrise through to sunset. One feature of the OH diurnal variation is a nocturnal layer in the 80-85 km region that is frequently, but not always, detectable in solar resonance for a short period following sunrise - the feature we label here as the "sunrise flash". This paper describes the observational analysis procedures involved in the quantitative measurement of the OSIRIS OH profiles together with a broad overview of the variability of the feature at sunrise. Also included is a photochemical model simulation of the OH sunrise layer using background atmospheric parameters, especially the water vapour mixing ratio, provided by the ACE/FTS instrument on the Canadian SCISAT satellite. For a number of nearly coincident measurements between OSIRIS OH and ACE/FTS water vapour, the model simulations show general agreement between the two. Agreement is improved by modifying the eddy mixing rates in the 80-85 km region, commensurate with the expected range of mixing rates.