The Mg II index for upper atmosphere modelling

The solar radio flux at 10.7 cm has been used in upper atmosphere density modelling because of its correla- tion with EUV radiation and its long and complete observa- tional record. A proxy, the Mg II index, for the solar chromo- spheric activity has been derived by Heath and Schlesinger (1986) from Nimbus-7 data. This index allows one to de- scribe the changes occurring in solar-activity in the UV Sun spectral irradiance. The use of this new proxy in upper at- mosphere 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 ex- ternal 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 possi- ble density model. determination to compute the atmospheric drag force. Pre- cise satellite orbit computation, for Earth observation as in case of oceanographic satellites, also requires the estimation of many density scale factors to absorb the errors in orbit pre- dictions induced by the density model. This is only possible when a tracking system is available, with stringent require- ments concerning the accuracy, and temporal and spatial con- tinuity of its observations. A more accurate drag estimate and consequently, a more realistic density model than what is available nowadays, is required to relax this operational constraint. The reasons for the slow progress in density mod- elling since the models as early as J71 (Jacchia, 1971) are the following, though not necessarily in this order:

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