Air density at heights near 180 km in 1968 and 1969, from the orbit of 1967-31a

Abstract

The satellite 1967-31A (A.T.S. 2), launched in April 1967, was unusual in having a low perigee (180 km initially) a near-constant cross-sectional area and a lifetime of more than two years. We have analysed its orbit to obtain 212 values of air density, mainly at heights between 160 and 190 km, during 14 months of high solar activity between 4 July 1968 and 2 September 1969 (when the satellite decayed). In general the air density exhibits only a weak dependence on solar activity, but the link between density and geomagnetic disturbances is obvious throughout: the two strongest geomagnetic storms, on 1 November 1968 and 15 May 1969, are accompanied by increases in density of 30 per cent and 70 per cent respectively. When such short-term variations are ignored, the underlying trend is a semi-annual variation in density, with maxima in October–November 1968 and March 1969, and minima in July 1968, January 1969 and July–August 1969. The July minima are deeper than the January minimum, and the average density at the maxima exceeds that at the minima by 32 per cent. A profile of density vs. height between 150 and 180 km for mid-1969 is also obtained; and a detailed comparison is made with previous results from 1968-59A.