  The solar F10.7 index is has been a reliable and sensitive activity index
since 1947. As with other indices, it has been showing unusual behavior in the
Cycle 23/24 minimum. The origins of the solar microwave flux lie in a variety
of features, and in two main emission mechanisms: free-free and gyroresonance.
In past solar cycles F10.7 has correlated well with the sunspot number SSN. We
find that this correlation has broken down in Cycle~23, confirming this with
Japanese fixed-frequency radiometric microwave data.
