A Re-analysis for the Observed Responses of Mesospheric H2O to Solar Cycle Forcings

This study reports on responses of mesospheric water vapor (H₂O) to the 11-year variations of the 10.7-cm solar flux proxy. The re-analyses herein are for time series of H₂O from the Halogen Occultation Experiment (HALOE) from June 1992 to November 2005, and they employ multiple linear regression (MLR) techniques. H₂O responses are large and negative in the upper mesosphere at solar maximum, due to the enhanced flux at Lyman-α wavelength. There is an observed changeover to weaker, positive H₂O responses for the middle and lower mesosphere, and there is good continuity for those responses across latitude and pressure-altitude. A positive response at solar maximum comes from enhancements of the uv-photolysis of molecular oxygen (O₂) leading to increased production of ozone and O (¹D), followed by the subsequent oxidation of methane (CH₄) and production of H₂O. Separate analyses verify that the responses of HALOE CH₄ are negative at solar maximum in the lowermost mesosphere. The domain of positive H₂O responses also exhibits asymmetry with latitude for the southern versus the northern hemispheres, most likely due to differences in the net transport across their domains. The associated H₂O trends from the MLR analyses agree with independent determinations of their trends in the mesosphere from satellite and ground-based observations over 1992-2005.