  The specification of the upper atmosphere strongly relies on solar proxies
that can properly reproduce the solar energetic input in the UV. Whilst the
microwave flux at 10.7 cm (also called F10.7 index) has been routinely used as
a solar proxy, we show that the radio flux at other wavelengths provides
valuable complementary information that enhances their value for upper
atmospheric modelling. We merged daily observations from various observatories
into a single homogeneous data set of fluxes at wavelengths of 30, 15, 10.7, 8
and 3.2 cm, spanning from 1957 to today. Using blind source separation (BSS),
we show that their rotational modulation contains three contributions, which
can be interpreted in terms of thermal bremsstrahlung and gyro-resonance
emissions. The latter account for 90% of the rotational variability in the
F10.7 index. Most solar proxies, such as the MgII index, are remarkably well
reconstructed by simple linear combination of radio fluxes at various
wavelengths. The flux at 30 cm stands out as an excellent proxy and is better
suited than the F10.7 index for the modelling the thermosphere-ionosphere
system, most probably because it receives a stronger contribution from thermal
bremsstrahlung. This better performance is illustrated here through comparison
between the observed thermospheric density, and reconstructions by the Drag
Temperature Model.
