An Explanation of the Differences Between the Sunspot Area Scales of the Royal Greenwich and Mt. Wilson Observatories, and the SOON Program
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Several studies have shown that the sunspot areas recorded by the Royal Greenwich Observatory (RGO) between 1874 – 1976 are about 40 – 50 % larger than those measured by the NOAA/USAF Solar Observing Optical Network (SOON) since 1966. We show here that while the two measurement sets provide consistent total areas for large spots, the impossibility of recording small spots as anything except dots in the SOON drawings leads to an underestimate of small spot areas. These are more accurately recorded by the RGO and other programs that use photographic or CCD images. The large number of such small spots is often overlooked. A similar explanation holds for the RGO umbral areas, which amount to 40 % more than those measured from Mt. Wilson data between 1923 and 1982. The neglected small spots have a much lower photometric contrast. Our explanation suggests, therefore, that the adjustment to spot irradiance blocking at the 1976 transition from RGO to SOON areas is smaller than the almost 50 % correction advocated by some recent, purely statistical, studies.
KeywordsTotal Solar Irradiance Small Spot Spot Area Photographic Plate Large Spot
I thank T. Baranyi, F. Clette, Pam Gilman, Peter Gilman, D. Hathaway, T. Henry, R. Howard, J. Kennewell, R. McFadden, P. McIntosh, and D. Willis for helpful discussions and data. I am especially grateful to Graham Appleby for his search at the Cambridge University Library for the RGO daily ledgers, and to T. Baranyi and C. Fröhlich for providing Figures 1 and 7. This study relied heavily on the comprehensive data archives maintained by the National Geophysical Data Center and by the Solar System Data Centre. The referee’s comments were very helpful. This work was supported by NASA grants NNX09AP96G and NNX10AC09G.