Title: Sodium Distribution in the Terrestrial Upper Atmosphere
Abstract: OBSERVATIONS1–3 of the twilight glow and dayglow have shown that there is a thin layer of free sodium atoms at a height of about 90 km. There is usually a daytime enhancement of the sodium abundance compared with the abundance at twilight; the daytime layer seems to be at the same height or a few kilometres higher than the twilight layer. Furthermore, the scale heights on the topside (and on the bottomside) of the daytime layer are unexpectedly small, ranging from 2.5 km (ref. 2) through 3 km (ref. 1) to 4.5 km (ref. 3), whereas the atmospheric scale height is about 6 km.