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Reconstruction of the Sunspot Group Number: The Backbone Method


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.

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  • Arlt, R.: 2008, Digitization of sunspot drawings by Staudacher, in 1749 – 1796. Solar Phys. 247, 399. DOI .

    Article  ADS  Google Scholar 

  • Asvestari, E., Usoskin, I.G.: 2016, Solar Phys., submitted.

  • Beck, R.: 1984, Zum Problem der Gruppeneinteilung von Sonnenflecken. SONNE – Mitteilungsblatt der Amateursonnenbeobachter 8, 64.

    Google Scholar 

  • Clette, F., Svalgaard, L., Vaquero, J.M., Cliver, E.W.: 2014, Revisiting the Sunspot Number, a 400-year perspective on the solar cycle. Space Sci. Rev. 186, 35. DOI .

    Article  ADS  Google Scholar 

  • Cliver, E.W., Ling, A.: 2016, The discontinuity in \({\sim\,}1885\) in the Group Sunspot Number. Solar Phys. DOI .

    Google Scholar 

  • Cliver, E.W., Clette, F., Svalgaard, L., Vaquero, J.M.: 2013, Recalibrating the Sunspot Number (SSN): The SSN workshops. Cent. Eur. Astrophys. Bull. 37(2), 401.

    ADS  Google Scholar 

  • Cliver, E.W., Clette, F., Svalgaard, L., Vaquero, J.M.: 2015, Recalibrating the Sunspot Number (SN): The 3rd and 4th SN workshops. Cent. Eur. Astrophys. Bull. 39, 1. ADS .

    ADS  Google Scholar 

  • Flamsted, J.: 1708, Observations of the solar eclipse, May 12, 1706. Phil. Trans. (London) 25, 2240.

    Google Scholar 

  • Foukal, P., Eddy, J.: 2007, Did the Sun’s prairie ever stop burning? Solar Phys. 245, 247. DOI .

    Article  ADS  Google Scholar 

  • Friedli, T.K.: 2009, Die Wolfsche Reihe der Sonnenfleckenrelativzahlen. Verbesserte Likelihood Ratio Tests zur Homogenitätsprüfung in strukturellen Zustandsraummodellen, Südwestdeutscher Verlag für Hochschulschriften, Saarbrücken.

  • Hoyt, D.V., Schatten, K.H.: 1998, Group sunspot numbers: A new solar activity reconstruction. Solar Phys. 181, 491.

    Article  ADS  Google Scholar 

  • Hubble, E.: 1929, A relation between distance and radial velocity among extra-galactic nebulae. Proc. Natl. Acad. Sci. USA 15(3), 168. DOI .

    Article  ADS  MATH  Google Scholar 

  • Kopecký, M., Růžičková-Topolová, B., Kuklin, G.V.: 1980, On the relative inhomogeneity of long-term series of sunspot indices. Bull. Astron. Inst. Czechoslov. 31, 267.

    ADS  Google Scholar 

  • Koyama, H.: 1985, Observations of Sunspots 1947 – 1984, Kawade Shobo Shinsha Publishers, Tokyo ISBN 4-309-25030-0.

    Google Scholar 

  • Leussu, R., Usoskin, I.G., Arlt, R., Mursula, K.: 2013, Inconsistency of the Wolf sunspot number series around 1848. Astron. Astrophys. 559, A28.

    Article  ADS  Google Scholar 

  • Lockwood, M., Nevanlinna, H., Barnard, L., Owens, M.J., Harrison, R.G., Rouillard, A.P., Scott, C.J.: 2014, Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr – Part 4: Near-Earth solar wind speed, IMF, and open solar flux. Ann. Geophys. 32, 383.

    Article  ADS  Google Scholar 

  • Owens, M.J., Lockwood, M.: 2012, Cyclic loss of open solar flux since 1868: The link to heliospheric current sheet tilt and implications for the Maunder minimum. J. Geophys. Res. 117, A04102.

    Article  ADS  Google Scholar 

  • Riley, P., Lionello, R., Linker, J.A., Cliver, E., Balogh, A., Beer, J., Charbonneau, P., Crooker, N., deRosa, M., Lockwood, M., Owens, M., McCracken, K., Usoskin, I., Koutchmy, S.: 2015, Inferring the structure of the solar corona and inner hemisphere during the Maunder minimum using global thermodynamic magnetohydrodynamic simulations. Astrophys. J. 802, 105. DOI .

    Article  ADS  Google Scholar 

  • Solanki, S.K., Usoskin, I.G., Kromer, B., Schüssler, M., Beer, J.: 2004, Unusual activity of the Sun during recent decades compared to the previous 11,000 years. Nature 431, 1084. DOI .

    Article  ADS  Google Scholar 

  • Stenflo, J.: 2014 Workshop Summary & Critique, 4th Sunspot Number Workshop, .

  • Svalgaard, L.: 2016, A Recount of Sunspot Groups on Historical Drawings. Solar Phys., in preparation.

  • Svalgaard, L., Cliver, E.W.: 2005, The IDV index: Its derivation and use in inferring long-term variations of the interplanetary magnetic field strength. J. Geophys. Res. 110, A12103. DOI .

    Article  ADS  Google Scholar 

  • Svalgaard, L., Cliver, E.W.: 2010, Heliospheric magnetic field 1835 – 2009. J. Geophys. Res. 115, A09111. DOI .

    Article  ADS  Google Scholar 

  • Svalgaard, L., Cliver, E.W., Le Sager, P.: 2003, Determination of interplanetary magnetic field strength, solar wind speed, and EUV irradiance, 1890 – 2003. In: Wilson, E. (ed.) Solar Variability as an Input to the Earth’s Environment, SP-535, ESA, Noordwijk, 15.

    Google Scholar 

  • Usoskin, I.G., Alanko-Huotari, K., Kovaltsov, G.A., Mursula, K.: 2005, Heliospheric modulation of cosmic rays: Monthly reconstruction for 1951 – 2004. J. Geophys. Res. 110, A12108.

    Article  ADS  Google Scholar 

  • Usoskin, I.G., Arlt, R., Asvestari, E., Hawkins, E., Käpylä, M., Kovaltsov, G.A., Krivova, N., Lockwood, M., Mursula, K., O’Reilly, J., Owens, M.J., Scott, C.J., Sokoloff, D.D., Solanki, S.K., Soon, W., Vaquero, J.M.: 2015, The Maunder minimum (1645 – 1715) was indeed a Grand minimum: A reassessment of multiple datasets. Astron. Astrophys. 581, A95. DOI . ADS .

    Article  ADS  Google Scholar 

  • Vaquero, J.M., Trigo, R.M.: 2014, Revised group Sunspot Number values for 1640, 1652, and 1741. Solar Phys. 289(3), 803. DOI .

    Article  ADS  Google Scholar 

  • Vaquero, J.M., Vazquez, M.: 2009, The Sun Recorded Through History, Astrophys. Space Sci. Library 361, Springer, Dordrecht ISBN 978-0-387-92789-3. DOI .

    Google Scholar 

  • Vaquero, J.M., Gallego, M.C., Usoskin, I.G., Kovaltsov, G.A.: 2011, Revisited sunspot data: A new scenario for the onset of the Maunder minimum. Astrophys. J. Lett. 731, L24.

    Article  ADS  Google Scholar 

  • Vaquero, J.M., Kovaltsov, G.A., Usoskin, I.G., Carrasco, V.M.S., Gallego, M.C.: 2015, Level and length of cyclic solar activity during the Maunder minimum as deduced from the active-day statistics. Astron. Astrophys. 577, A71.

    Article  ADS  Google Scholar 

  • Waldmeier, M.: 1938, Chromosphärische Eruptionen. I. Z. Astrophys. 16, 276.

    ADS  Google Scholar 

  • Waldmeier, M.: 1948, 100 Jahre Sonnenfleckenstatistik, Astron. Mitt. Eidgenöss. Sternwarte Zür. 16, No. 152, 1

  • Waldmeier, M.: 1955: Ergebnisse und Probleme der Sonnenforschung, Geest & Portig, 2. erweiterte Aufl. Leipzig.

  • Willis, D.M., Coffey, H.E., Henwood, R., Erwin, E.H., Hoyt, D.V., Wild, M.N., Denig, W.F.: 2013, Solar Phys. 288, 117. DOI .

    Article  ADS  Google Scholar 

  • Woeckel, L.: 1846, Die Sonne und ihre Flecken, Verlag Friedrich Campe, Nürnberg.

    Google Scholar 

  • Young, C.A.: 1881, The Sun, Appleton, New York.

    Google Scholar 

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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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Sunspot Number Recalibration

Guest Editors: F. Clette, E.W. Cliver, L. Lefèvre, J.M. Vaquero, and L. Svalgaard

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Svalgaard, L., Schatten, K.H. Reconstruction of the Sunspot Group Number: The Backbone Method. Sol Phys 291, 2653–2684 (2016).

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  • Solar Cycle
  • Reconstruction
  • Sunspot Group Number