The orbit of Echo 2 is used to derive values of exospheric density at heights near 1100 km during 1967. The results are combined with those from two earlier studies to show the behaviour of the semi-annual variation at solar minimum and throughout the entire rising phase of the current solar cycle. The July minima are consistently deeper than the January minima, while there is no systematic difference between the April and October maxima. It appears that the ratio of the mean maximum density to the mean minimum in the semi-annual variation decreased from about 2.5 in 1964 to about 1.8 (or possibly even lower) in 1966, as the 10.7 cm radiation flux increased from 70 to 120 × 10-22 W m-2 Hz-1, and the ratio then increased slightly to about 2.0 in 1967. © 1969.
