Number densities of atomic oxygen and molecular nitrogen in the thermosphere

Abstract

The absorption by the Earth's atmosphere of the solar ultraviolet lines at 625 Å (Mg X), 630 Å(O V), 770 Å (Ne VIII) and 765 Å(N IV) was observed by instruments on the satellite OSO-IV. We calculate the absorption curves using a static diffusion model of the atmosphere (Jacchia, 1965) and determine the number densities of atomic oxygen and molecular nitrogen at 120 km by fitting to the observations. We demonstrate that virtually the same theoretical absorption curves can arise from two models in which the number densities of the individual species differ substantially. Comparison of absorption curves at a common wave-length indicates that the response of the exospheric temperature to a change in geomagnetic activity exceeds 8 hr, and that the sunset to sunrise change in exospheric temperature exceeds that derived by Jacchia. The number densities of atomic oxygen obtained from the 625 Å and 630 Å line absorptions agree with either the 1965 or the most recent static diffusion model, depending on which of the published cross sections is adopted, whereas those derived from 765 Å and 770 Å agree with neither model. The derived number densities of molecular nitrogen contradict the old model but seem consistent with the recent model.