Upper-atmosphere zonal winds from satellite orbit analysis: An update

Abstract The rotation rate Λ of the upper atmosphere can be evaluated from the observed changes in satellite orbital inclination, and in 1983 we reviewed the results then available. From the 85 values accepted for the review, it was possible to determine the variation with height (130–680 km), local time and season; but dependence on other parameters was not established. The present paper updates the previous review, incorporating 18 new values determined in the past 5 years and generally of better accuracy. The previous picture is strengthened by the new data but not significantly changed. After adding the new results, we find that the average value of Λ increases from 1.0 rev day −1 at 125 km height to about 1.2 at 300 km, and then decreases to about 0.9 at 500 km; Λ is higher in the evening (18–24 h), with a maximum value near 300 km which corresponds to a West-to-East wind of 150 m s −1 , and is lower in the morning (06–12 h), with East-to-West winds of order 50 m s −1 at heights of 200–400 km. The values of Λ are higher in winter than in summer for heights of 200–300 km, and there is an indication that Λ may vary with solar activity, being about 0.1 lower for medium solar activity than for high or low solar activity. Results for heights above 450 km are still sparse.