Some aspects of a global thermospheric density model deduced from the analysis of the orbit of Intercosmos 13 rocket (1975-22B)

Abstract

An empirical density formula is explored as a practical model for atmospheric variations and satellite drag analyses. Expanding neutral air density as a series of spherical harmonics and normalizing to a fixed height, an analytical expression for the rate of change of the mean motion is developed for an oblate atmosphere with density scale height varying linearly with altitude. A subset of the coefficients in the density expansion is determined by least-squares adjustment to the observed orbital decay rate of Intercosmos 13 rocket (1975-22B) for the period May 1975–December 1979. Comparisons against four thermospheric models are undertaken for the solar activity effect and the diurnal and semi-annual variations. Given the even spread of data and the increase in solar activity from low to moderate, the air density variation with solar activity is particularly well determined. The results support the “J77” model revealing a greater increase in density with the daily solar index than either the “MSIS” or “DTM” thermospheric models near the solar minimum. Analyses of the diurnal and semi-annual variations are less exact.