Abstract
Abstract. The solar radio flux at 10.7 cm has been used in upper atmosphere density modelling because of its correlation with EUV radiation and its long and complete observational record. A proxy, the Mg II index, for the solar chromospheric activity has been derived by Heath and Schlesinger (1986) from Nimbus-7 data. This index allows one to describe the changes occurring in solar-activity in the UV Sun spectral irradiance. The use of this new proxy in upper atmosphere density modelling will be considered. First, this is supported by the 99.9% correlation between the solar radio flux (F10.7) and the Mg II index over a period of 19 years with, however, large differences on time scales of days to months. Secondly, correlation between EUV emissions and the Mg II index has been shown recently, suggesting that this last index may also be used to describe the EUV variations. Using the same density dataset, a model was first run with the F10.7 index as a solar forcing function and second, with the Mg II index. Comparison of their respective predictions to partial density data showed a 3–8% higher precision when the modelling uses the Mg II index rather than F10.7. An external validation, by means of orbit computation, resulted in a 20–40% smaller RMS of the tracking residuals. A density dataset spanning an entire solar cycle, together with Mg II data, is required to construct an accurate, unbiased as possible density model.Key words. Atmospheric composition and structure (middle atmosphere – composition and chemistry; thermosphere – composition and chemistry) – History of geophysics (atmospheric sciences)
Highlights
Thermosphere models, representing temperature and density as a function of environmental parameters, are used in upper atmosphere studies, and in satellite orbit determination to compute the atmospheric drag force
The Dynamics Explorer 2 (DE-2) mass spectrometer does not provide absolute densities; the use of its measurements require scaling by a calibration factor. These have not been derived in this study, but the scaling factors that were used in the construction of DTM94 were taken
Comparison of the test models DTM-Mg II and DTM-F107 demonstrated the superior representativity of the Mg II index with respect to upper atmosphere heating processes, in particular, under high solar activity conditions
Summary
Thermosphere models, representing temperature and (partial) density as a function of environmental parameters, are used in upper atmosphere studies, and in satellite orbit determination to compute the atmospheric drag force. For Earth observation as in case of oceanographic satellites, requires the estimation of many density scale factors to absorb the errors in orbit predictions induced by the density model. This is only possible when a tracking system is available, with stringent requirements concerning the accuracy, and temporal and spatial continuity of its observations. The sparse partial and total density, and temperature data available with their own precision and accuracy. Large regions of the upper atmosphere remain sparsely observed, notably in the 100–200 km altitude range
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