Air density at heights of 140–180 km, from analysis of the orbit of 1968-59A

Abstract

The satellite 1968-59A was launched on 11 July 1968 into a polar orbit with an initial perigee height of 160 km and apogee height 1800 km. The satellite did not decay until 6 November 1968, and analysis of its orbit yields values of air density at average intervals of 2 days between July and early November, mainly at heights of 140–180 km. Separate profiles of air density vs. height are obtained for July–August and for September–October 1968, with the latter giving values of density about 10 per cent higher. This increase in density reveals the existence of the semi-annual variation in density, well known at greater heights: the maximum density in October 1968 was about 20 per cent higher than the minimum in July, for heights near 170 km. When corrected to a fixed height, the density exhibits variations that are correlated with the geomagnetic index K p . The influence of solar radiation is only of minor importance and there is no evidence of variations with latitude or between day and night. The variations in density given by 1968-59A are found to be in good agreement with results from satellites at other heights.