On the response of the O(1S) dayglow emission rate to the Sun's energy input: An empirical model deduced from WINDII/UARS global measurements

Abstract

More than 520,000 emission rate profiles of the O(1S) dayglow (557.7 nm, the atomic oxygen green line) were measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmospheric Research Satellite (UARS) during 1991–1997, providing an unprecedented and unique resource for studying the O(1S) emission layer, its related physics and chemistry, and the response of the mesosphere and thermosphere to the solar input. The daytime O(1S) emission is one of the most remarkable and persistent phenomena in the Earth's atmosphere between 80 and 280 km. The emission has two components, peaking at 140–180 km and 94–104 km. WINDII measurements show that both components are sensitively correlated with the Sun in the sense that (1) for a given day, the peak emission rates in both the F‐region and the E‐region increase with increasing cosine of the solar zenith angle, the height of the peak emission rate in the F‐region decreases with increasing peak emission rate but that in the E‐region does not have a clear relationship with the peak emission rate; (2) both the peak emission rate and its height in the F‐region as well in the E‐region increase with increasing solar irradiance, i.e., they follow the solar cycle. For the first time an empirical model of the green line emission rate and height of the peak emission are derived from WINDII global measurements over six years as functions of the solar zenith angle and the solar irradiance using the daily solar F10.7 cm flux as a proxy. This model provides a baseline for the unperturbed daytime green line emission for any time and any location. Owing to the direct effect of the solar zenith angle, the emission rate is symmetrical with respect to local noon; and globally, it is symmetrical with respect to the equator at equinox, and is much larger in the summer hemisphere than in the winter hemisphere at solstice. As a result, for a constant solar irradiance, the emission rate has an annual oscillation at mid and high latitudes, and a semiannual oscillation at the equator. In the solar cycle 22, for an overhead Sun, the max/min ratio of the F10.7 cm flux is about four, and the max/min ratio of the integrated O(1S) emission rate is about three. Some unsolved problems regarding the mechanism of the green line emission are reported.