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

The nitric oxide (NO) density is of principal importance for the lower iono sphere as it is the source of the main ionized component. The mesospheric N O density climatology based on HALOE/UARS measurements (Siskind et al., Adv ances 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 A tmospheric and Solar-Terrestrial Physics 60 (1998) 1445-1457) revealed, amo ng 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 t he entire daytime. We analyzed multi-frequency radio wave absorption measur ements in the lower ionosphere over Central Europe, partly together with so lar Lyman-a and X-ray fluxes, in order to estimate the NO density. The resu lts show that our "ionospheric" NO densities are comparable to, or somewhat higher than, the climatological NO densities of Siskind et al. (1998). The y also show that the diurnal asymmetry in summer is a stable and regular fe ature of the lower ionosphere throughout the daytime, and that there is a s ubstantial diurnal asymmetry in the NO density in the upper middle atmosphe re that coincides with that revealed by the HALOE data and which is respons ible for the asymmetry in the lower ionosphere. (C) 2000 Elsevier Science L td. All rights reserved.