Solar Physics

, Volume 289, Issue 5, pp 1517–1529 | Cite as

An Explanation of the Differences Between the Sunspot Area Scales of the Royal Greenwich and Mt. Wilson Observatories, and the SOON Program

  • Peter FoukalEmail author


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.


Total Solar Irradiance Small Spot Spot Area Photographic Plate Large Spot 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



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.


  1. Allen, C.: 1964, Astrophysical Quantities, 2nd edn. Athlone Press, London, 180. Google Scholar
  2. Balmaceda, L., Solanki, S., Krivova, N., Foster, S.: 2009, J. Geophys. Res. 114, 7104. CrossRefGoogle Scholar
  3. Baranyi, T., Kiraly, S., Coffey, H.: 2013, Mon. Not. Roy. Astron. Soc. 434, 1713. doi: 10:1093/mnras/stt1134. ADSCrossRefGoogle Scholar
  4. Baranyi, T., Györi, L., Ludmány, A., Coffey, H.: 2001, Mon. Not. Roy. Astron. Soc. 323, 223. ADSCrossRefGoogle Scholar
  5. Fligge, M., Solanki, S.: 1997, Solar Phys. 173, 427. ADS:1997SoPh..173..427F, doi: 10.1023/A:1004971807172. ADSCrossRefGoogle Scholar
  6. Fontenla, J., Avrett, E., Thuillier, G., Harder, J.: 2006, Astrophys. J. 639, 441. ADSCrossRefGoogle Scholar
  7. Foukal, P., Lean, J.: 1986, Astrophys. J. 302, 836. ADSCrossRefGoogle Scholar
  8. Foukal, P., Bernasconi, P.: 2008, Solar Phys. 248, 1. ADS:2008SoPh..248....1F, doi: 10.1007/s11207-008-9134-7. ADSCrossRefGoogle Scholar
  9. Fröhlich, C.: 2011, Contrib. Astron. Obs. Skaln. Pleso 41, 113. ADSGoogle Scholar
  10. Gerlei, O.: 1987, Publ. Debrecen Heliophysical Obs., Heliographic Ser. No. 1. Google Scholar
  11. Györi, L.: 1998, Solar Phys. 180, 109. ADS:1998SoPh..180..109G, doi: 10.1023/A:1005081621268. ADSCrossRefGoogle Scholar
  12. Hathaway, D., Wilson, R., Reichmann, E.: 2002, Solar Phys. 211, 357. ADS:2002SoPh..211..357H, doi: 10.1023/A:1022425402664. ADSCrossRefGoogle Scholar
  13. Hohenkerk, C., Ladley, C., Rudd, P.: 1967, Royal Observatory Annals 11. Photoheliograph Results. Google Scholar
  14. Howard, R., Gilman, P., Gilman, P.: 1984, Astrophys. J. 283, 373. ADSCrossRefGoogle Scholar
  15. Hoyt, D., Eddy, J., Hudson, H.: 1983, Astrophys. J. 275, 878. ADSCrossRefGoogle Scholar
  16. Moran, T., Foukal, P., Rabin, D.: 1992, Solar Phys. 142, 35. ADS:1992SoPh..142...35M, doi: 10.1007/BF00156632. ADSCrossRefGoogle Scholar
  17. Parker, E.: 1995, Astrophys. J. 440, 415. ADSCrossRefGoogle Scholar
  18. Steinegger, M., Vazquez, M., Bonet, J., Brandt, P.: 1996, Astrophys. J. 461, 478. ADSCrossRefGoogle Scholar
  19. Wilson, P., McIntosh, P.: 1969, Solar Phys. 10, 370. ADS:1969SoPh...10..370W, doi: 10.1007/BF00145525. ADSCrossRefGoogle Scholar
  20. Wittmann, A., Schröter, E.: 1969, Solar Phys. 10, 357. ADS:1969SoPh...10..357W, doi: 10.1007/BF00145524. ADSCrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2013

Authors and Affiliations

  1. 1.Heliophysics, Inc.NahantUSA

Personalised recommendations