THE CURIOUS CASE OF THE GREENWICH FACULAE

We analyze the record of facular areas compiled by the Royal Greenwich Observatory (RGO) from daily white-light observations between 1874 and 1976. Curiously, the relative amplitudes of the three largest sunspot cycles 17, 18, and 19 in this record are reversed when they are ranked by facular area. We show that this negative correlation arises from a general decrease of the ratio A(F)/A(S), of facular to sunspot area, with increasing As. Within a given cycle, A(F)/A(S) decreases in active regions of large A(S), but A(F)/A(S) is also lower at all A(S), in cycles of higher peak amplitude in A(S). This decrease of A(F)/A(S) in large spot groups is consistent with its decrease in younger, more active solar-mass stars, and it may explain why stars only slightly more magnetically active than the Sun tend to exhibit much greater variability in broad-band photometry. We suggest that the physical explanation is an increased spatial filling factor of magnetic Aux, favoring formation of sunspots over faculae. We also explain why the decrease in A(F)/A(S) is not seen in the disc-integrated Ca K plage areas, nor in the F10.7 microwave index, both of which exhibit remarkable linearity when plotted against smoothed sunspot area. This explanation suggests how complementary data on faculae and plages from RGO and Mt. Wilson could be used to improve empirical models of total irradiance variation, extending back to 1874.