Response of Middle Atmospheric Hydroxyl Radical to the 27-day Solar Forcing

In this study, we use the Aura Microwave Limb Sound (MLS) data to examine the response of middle atmospheric hydroxyl radicals (OH) to the 27-day solar rotational variability during MLS's first measurement year (2004–2005) when solar activities are moderately strong. The results are compared to the simulations of the 1-D photochemical model KINETICS using different UV variabilities. Shapiro et al. [2012] have examined the 27-day solar cycle modulation in tropical mesospheric OH measured by MLS, and found that the OH 27-day solar response is ~1% per 1% change in Lyman-alpha at 80 km. However, as different band-pass filters are applied, the OH response varies significantly. An "optimal" filter is selected based on the convergence of OH response with respect to filter window size. The corresponding OH response to Lyman-alpha is examined for various altitudes and latitudes. The results suggest that in the middle atmosphere model simulations underestimate OH variabilities to short term solar forcing by 20% - 50%, which cannot be explained by the small uncertainties in solar spectral irradiance (SSI) variability.