Response of the O(1S) Dayglow to the Solar Zenith Angle and Solar Irradiance: An Empirical Model by WINDII on UARS
Shengpan P. Zhang [sheng@stpl.cress.yorku.ca] and Gordon G. Shepherd, CRESS, York University, Toronto, Canada

Absorption of solar Extreme Ultra Violet (EUV) radiation in the middle thermosphere and hydrogen Lyman-beta radiation in the lower thermosphere produces "dayglow" emission from the atomic oxygen O(1S) level at 557.7 nm. The emission rate of the airglow is a sensitive measure of the thermospheric response to solar input. More than 520,000 O(1S) dayglow profiles were observed by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) during 1991-1997, providing an unprecedented and unique database for studying the mechanisms and global climatology of the dayglow layers, and the energy transfer processes from the solar radiation into the mesosphere and lower thermosphere. For the first time, an empirical model of the emission rate is derived from those measurements as function of the solar zenith angle and solar irradiance using the F10.7 cm flux as a proxy, which serves as a baseline for the unperturbed emission rate. The daytime O(1S) emission rate profile has two peaks at about 150 km and 100 km of altitude, respectively. During the declining phase of solar cycle 22 (DEC91-APR97) for an overhead Sun the maximum volume emission rates (photon cm-3 s-1) reduced from about 1100 to 450 at about 150 km, and from 1700 to 520 at about 100 km. Dramatic enhancements of the emission rate from the baseline are frequently observed at high latitudes as the daytime aurora.