Thermospheric densities derived from spacecraft orbits: Application to the Starshine satellites

[1] The amplitude and variability of the total mass density of the Earth's atmosphere at altitudes between 200 and 475 km are analyzed for the period from June 1999 to January 2003, around the maximum of solar activity in cycle 23. The densities are derived with uncertainties of order +/- 6% from analysis of the approximately circular orbits of three Starshine spacecraft of known ballistic coefficients. Local densities averaged over all three Starshine missions are similar to 4% lower than the corresponding NRLMSISE-00 model values, which may reflect a secular decrease from thermospheric cooling. Differences of order +/- 15% occur on timescales of days to weeks, and larger differences of as much as 30% can persist on timescales of several months. Some differences are traceable to the NRLMSIS model's use of the 10.7 cm proxy of EUV radiation, since they are reduced when NRLMSIS is evaluated using the Mg II index, which is a better proxy of variations in EUV irradiance that heats the thermosphere. Comparisons of 27-day density cycles extracted by complex demodulation indicate that NRLMSIS underestimates upper atmospheric density increases associated with solar rotational modulation of EUV radiation by as much as a factor of two.