Abstract
The orbit of the satellite Midas 2 (1960 ζ) is analysed to determine the variations in air density at a height of 480 km between November 1965 and November 1967. After correction for variations in solar activity, the density exhibits a semi-annual variation with the maximum monthly values (April and October) exceeding the minimum (January and June) by a factor of 2.1. This variation is appreciably greater than indicated by existing model atmospheres. The form of the variations is similar to that obtained (up to January 1967) from Echo 2 at 1130 km and (up to June 1967) from Secor 6 at 191 km. The results suggest that the last three ‘troughs’ in the semi-annual variation (in mid-1966, early 1967 and mid-1967) have been double minima, the main January and July minima being accompanied by distinct subsidiary minima in March and May.
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