The semi-annual variation in air density for 1974–1978 from the orbit of 1972-05B

Values of air density at 712 epochs between August 1973 and September 1978 have been determined using orbital elements of 1972-05B with orbital decay rates from NORAD bulletins. Normalised to a series of fixed heights and cleared of the effects of solar activity, geomagnetic activity and the diurnal variation, the residual air density was analysed for the semi-annual variation. This variation exhibited maxima usually in April and October and minima usually in January and July.

For 1974–1978, this study revealed near-identical values of the April and October maxima and a July minimum 12% stronger than the January minimum. Further, the shape and phase of the variation exhibited an irregular pattern from year-to-year. Overall the amplitude of the variation was considerably greater than that given in the atmospheric models (CIRA, 1972; Jacchia, 1977). Other observations included the presence of subsidiary minima and maxima in late May and June respectively during 1977 and 1978 and a general increase in air density from mid 1977 onwards, relative to the atmospheric models.