Abstract
The magnitude of the effective eddy diffusion coefficient and its height profile are crucial to the composition structure near and above turbopause in the lower thermosphere. However, neither theories nor direct observations are available for the eddy diffusion coefficient in the upper atmosphere. This paper considers theoretically the effects of the profile of the effective eddy diffusion coefficient on the height variations in O, O 2 and He number densities. Assuming various profiles of the eddy diffusion coefficient based on some theoretical and experimental evidence, the time-dependent continuity equations have been solved, including transport terms by molecular and eddy diffusion as well as various chemical reactions. The results indicate that a maximum eddy diffusion coefficient of the order of 10 7 cm 2 sec −1 in the height range of 95–105 km would explain most observations of the height variation in composition by rocket-borne mass spectrometer, except for He, where the lateral flow of particles in ballistic trajectories in the exosphere may produce a large seasonal variation. It is shown that the reduction of [O] is particularly pronounced when the eddy diffusion coefficient has a peak near 100 km. The result also shows, in accordance with keneshea and Zimmerman (1970), that the local equilibrium is not maintained, but that the total production and loss for the atomic oxygen over a 24-hr period is almost in balance at each height.
Published Version
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