Diurnal Variation as a Constraint on SABER Mesospheric Atomic Oxygen

Several studies have presented methods for estimating daytime or nighttime atomic oxygen (O) from measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite. In the present study, we describe algebraic formulas adapted from the standard SABER Version 2 method for estimating daytime and nighttime O based on the assumption that ozone in the upper mesosphere is in equilibrium. We highlight uncertainties in the calculations that lead to global mean differences ranging up to a factor of two or more in the nighttime O concentration. Important considerations are the choice of which reactions are included in the ozone budget and the values of uncertain parameters used in the model of emissions from vibrationally excited hydroxyl. SABER observations cover almost all local times. Since O is long-lived in the upper mesosphere, the dominant source of diurnal variation is transport by tidal winds. Even when accounting for this transport, there are appreciable differences between daytime and nighttime O. Daytime O is higher than is consistent with the nighttime O that is estimated from several methods. The estimated daytime O has a strong and nearly linear dependence on ozone while the nighttime O is sensitive to multiple parameters in the model of OH airglow emissions. The day/night discrepancy supports other evidence that the SABER daytime mesospheric ozone has large uncertainties and needs to be updated.