The MgII index for upper atmosphere modelling

The solar radio flux at 10.7 cm has been used in upper atmosphere density m odelling because of its correlation with EUV radiation and its long and com plete observational record. A proxy, the Mg II index, for the solar chromos pheric activity has been derived by Heath and Schlesinger (1986) from Nimbu s-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 up per atmosphere density modelling will be considered. First, this is support ed 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 tim e scales of days to months. Secondly, correlation between EUV emissions and the Mg II index has been shown recently, suggesting that this last index m ay also be used to describe the EUV variations. Using the same density data set, 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 predictio ns to partial density data showed a 3-8% higher precision when the modellin g 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 resi duals. A density dataset spanning an entire solar cycle, together with Mg I I data, is required to construct an accurate, unbiased as possible density model.