Upper-atmosphere zonal winds from satellite orbit analysis

We review and interpret the values of upper-atmosphere rotation rate (zonal winds) obtained by analyzing satellite orbits determined from observations. The history of the method is outlined, the basic principles are explained, objections to the method are answered, and three examples are given. Existing analyses of the atmospheric rotation rate A are critically reviewed. It is indicated that the value of A (in rev/day), averaged over both local time and season, increases from 1.0 at 125 km to 1.22 at 325 km and then decreases to 1.0 at 430 km and 0.82 at 600 km. The value of A is higher in the evening (18-24 h), with a maximum value (near 1.4) corresponding to a west-to-east wind of 150 m/s at heights near 300 km. The value of A is lower in the morning (06-12 h), with east-to-west winds of order 50 m/s at heights of 200-400 km. There is also a consistent seasonal variation, the values of A being on average 0.15 higher in winter and 0.1 lower in summer than the average seasonal value. No significant variation with solar activity is found, but there is a slight tendency for a greater rotation rate at lower latitudes for heights above 300 km. Unexpectedly, the values for the 1960s are found to be significantly higher than those for the 1970s. Finally, these observational values are compared with the theoretical global model of Fuller-Rowell and Rees. Some differences in the mean values are found.