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The specification of the upper atmosphere strongly relies on solar proxiesthat can properly reproduce the solar energetic input in the UV. Whilst themicrowave flux at 10.7 cm (also called F10.7 index) has been routinely used asa solar proxy, we show that the radio flux at other wavelengths providesvaluable complementary information that enhances their value for upperatmospheric modelling. We merged daily observations from various observatoriesinto a single homogeneous data set of fluxes at wavelengths of 30, 15, 10.7, 8and 3.2 cm, spanning from 1957 to today. Using blind source separation (BSS),we show that their rotational modulation contains three contributions, whichcan be interpreted in terms of thermal bremsstrahlung and gyro-resonanceemissions. The latter account for 90% of the rotational variability in theF10.7 index. Most solar proxies, such as the MgII index, are remarkably wellreconstructed by simple linear combination of radio fluxes at variouswavelengths. The flux at 30 cm stands out as an excellent proxy and is bettersuited than the F10.7 index for the modelling the thermosphere-ionospheresystem, most probably because it receives a stronger contribution from thermalbremsstrahlung. This better performance is illustrated here through comparisonbetween the observed thermospheric density, and reconstructions by the DragTemperature Model.

Synoptic solar radio observations as proxies for upper atmosphere modelling