Solar Physics

, Volume 291, Issue 9–10, pp 2653–2684 | Cite as

Reconstruction of the Sunspot Group Number: The Backbone Method

  • Leif SvalgaardEmail author
  • Kenneth H. Schatten
Sunspot Number Recalibration


We have reconstructed the sunspot-group count, not by comparisons with other reconstructions and correcting those where they were deemed to be deficient, but by a re-assessment of original sources. The resulting series is a pure solar index and does not rely on input from other proxies, e.g. radionuclides, auroral sightings, or geomagnetic records. “Backboning” the data sets, our chosen method, provides substance and rigidity by using long-time observers as a stiffness character. Solar activity, as defined by the Group Number, appears to reach and sustain for extended intervals of time the same level in each of the last three centuries since 1700 and the past several decades do not seem to have been exceptionally active, contrary to what is often claimed.


Solar Cycle Reconstruction Sunspot Group Number 



We thank Ed Cliver for stimulating discussions and for his hospitality during Svalgaard’s several (working) visits to NSO at Sunspot, NM. We thank Frédéric Clette and the team at SILSO for access to their database. Rainer Arlt graciously provided access to photographs of Staudach’s drawings, now held at the library of the Astrophysikalisches Institut, Potsdam. We thank Ilya Usoskin for permission to use data for Figure 37. We also thank the Locarno observers for their hospitality during our visits, for their effort to share their data with us, and for implementing an additional un-weighted sunspot counting procedure going forwards. We acknowledge with pleasure the use of drawings from Specola Solare Ticinese, Locarno ( ). Some sunspot drawings were provided by the Kanzelhöhe Observatory, University of Graz, Austria ( ). This study includes data from the synoptic program at the 150-Foot Solar Tower ( ) of the Mt. Wilson Observatory, operated by UCLA, with funding from NASA, ONR, and NSF, under agreement with the Mt. Wilson Institute. This research has made use of NASA’s Astrophysics Data System. L. Svalgaard thanks Stanford University for support.


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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.HEPLStanford UniversityStanfordUSA
  2. 2.a.i. solutionsLanhamUSA

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