Nitric oxide densities and their diurnal asymmetry in the upper middle atmosphere as revealed by ionospheric measurements

Abstract

The nitric oxide (NO) density is of principal importance for the lower ionosphere as it is the source of the main ionized component. The mesospheric NO density climatology based on HALOE/UARS measurements (Siskind et al., Advances in Space Research 21 (1998) 1353–1362) and a comparison of the HALOE NO density data with some ionospheric data (Friedrich et al., Journal of Atmospheric and Solar-Terrestrial Physics 60 (1998) 1445–1457) revealed, among others, a large “puzzling” diurnal asymmetry of the NO density. In this paper, the existence of a diurnal asymmetry of the NO density is confirmed by independent data and is extended from the sunrise/sunset HALOE data to the entire daytime. We analyzed multi-frequency radio wave absorption measurements in the lower ionosphere over Central Europe, partly together with solar Lyman- α and X-ray fluxes, in order to estimate the NO density. The results show that our “ionospheric” NO densities are comparable to, or somewhat higher than, the climatological NO densities of Siskind et al. (1998). They also show that the diurnal asymmetry in summer is a stable and regular feature of the lower ionosphere throughout the daytime, and that there is a substantial diurnal asymmetry in the NO density in the upper middle atmosphere that coincides with that revealed by the HALOE data and which is responsible for the asymmetry in the lower ionosphere.