Temporal variation of the ratio between the monthly mean F_10.7 flux and a constant proxy relation (4th-order polynomial) (upper panel), after subtracting a 67 sfu quiet-Sun base flux. The same ratio for the yearly mean F_10.7 flux is shown in the lower panel. The points near cycle maxima and minima are colored in blue and red respectively. The monthly means show a slight solar-cycle modulation, and both curves show the 1980 jump between two stable periods.