Molecular oxygen density and Lyman-α absorption in the upper atmosphere

Rocket measurements of the absorption of solar ultraviolet radiation provide a powerful method for the determination of the composition of the upper atmosphere. The method is particularly suited to the determination of the density distribution of molecular oxygen above 60 km, and the results of rocket experiments in the northern hemisphere have been conveniently summarized by Jursa et al. [1963] and compared with the O2 distributions suggested by Miller [1957] and by Nicolet [1959]. We have made measurements from Woomera, Australia (lat. 30°56′S, long. 136°31′E), of the absorption of solar Lyman-α radiation (1216 A) in order to determine the O2 density above 70 km. Nitric oxide filled ion chambers with lithium fluoride windows have been used as the radiation detectors. The detectors are similar in type to those described by Friedman [I960]. The chambers have a spectral response which extends from 1050 to 1350 A, but, owing to the high intensity of the Lyman-α line in this region of the solar spectrum and the narrow window in the O2 absorption cross section at 1216 A, the chambers may be considered almost pure Lyman-α detectors for the present purpose. A half meter vacuum ultraviolet monochrometer was used to calibrate the detectors in the laboratory, the absolute sensitivities being determined by comparison with a standard parallel plate ion chamber filled with nitric oxide; the photoionization yield of nitric oxide for Lyman-α radiation was taken to be 81% [Watanabe and Matsunaga, 1961].