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The observed SSN divided by a synthetic SSN computed from a polynomial fit to the 10.7 flux over the interval 1951–1990. Red points are SIDC sunspot numbers while blue points are SWPC (NOAA) sunspot numbers scaled down by the their average k-factor of 0.655. Only years when the SSN was above 10 are included. Since ~ 1990 the observed SSN is progressively lower for a given 10.7 flux.
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