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

Abstract

[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 $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.

Thermospheric Density and Wind Determination from Satellite Dynamics

“…Ballistic coefficients were calculated under the assumption that the long-term average of the ratio of observed to modeled density ratios should equal one. The resulting ballistic coefficient values were generally found to be within 5% of values found in other research [Bowman, 2002;Lean et al, 2006;Emmert et al, 2006].…”

Thermospheric mass density: A review

“…These measurements indicate that other solar activity indices are better proxies for short-term EUV variations than F 10.7 ; in particular, the MgII index (Viereck et al, 2001), which represents a spectral feature in the FUV band near 280 nm, is superior to F 10.7 at all EUV wavelengths longer than ~27 nm (Lean et al, 2011a). Correspondingly, MgII has been found to be a better indicator of short-term (< 81 days) density variations than F 10.7 Lean et al, 2006). Fig.…”

“…7. Lean et al (2006) also found a weaker 27 day density response using F 10.7 , compared to using MgII.…”

The 30 cm radio flux as a solar proxy for thermosphere density modelling

“…This result is important for satellite drag applications because of the high variability of the thermospheric density at those time scales. In addition, the MgII index has been found to be a better proxy for the 27-day density response than F10.7 (Lean et al 2006) and so we may expect F30 to be relevant as well for the density. This will be investigated in the next section.…”

“…Those proxies which have been specifically recommended for drag modelling (e.g. Bowman et al 2006) are: -F10.7, which is used in all thermosphere models; -the MgII core-to-wing index, which is used in many studies for the forcing of the upper atmosphere (Lean et al 2006) and also in the DTM2000 thermosphere model (Bruinsma et al 2003). We use the Bremen composite 3 , whose record starts on 7 November 1978.…”