Revisiting the Sunspot Number
Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (Sol. Phys. 179:189–219, 1998a, 181:491–512, 1998b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration.
After retracing the construction of those two composite series, we present the new concepts and methods used to self-consistently re-calibrate the original sunspot series. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880–1915 for the Group Number and in 1947 and 1980–2014 for the Sunspot Number.
After establishing those new corrections, we then consider the implications on our knowledge of solar activity over the last 400 years. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18th century, solar activity had already returned to levels equivalent to those observed in recent solar cycles in the 20th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21st century.
KeywordsSun Sunspots Sunspot Number Solar cycle Solar activity
Part of the work contributed here by F. Clette was developed in the framework of the TOSCA project (ESSEM COST action ES1005 of the European Union; http://lpc2e.cnrs-orleans.fr/~ddwit/TOSCA/Home.html), of the SOLID project (EU 7th Framework Program, SPACE collaborative projects; http://projects.pmodwrc.ch/solid/) and of the Solar-Terrestrial Center of Excellence (http://www.stce.be). F. Clette would like to acknowledge the personal contributions from Laure Lefèvre and Laurence Wauters.
L. Svalgaard acknowledges support from Stanford University.
J.M. Vaquero acknowledges support from the Junta de Extremadura (Research Group Grant No. GR10131) and the Ministerio de Economía y Competitividad of the Spanish Government (AYA2011-25945).
The authors also wish to thank Sergio Cortesi, Marco Cagnotti and Michele Bianda for their hospitality and help for our various investigations on the Locarno and Zürich observations.
The Sunspot Number Workshop series has benefitted from the support of the Royal Observatory of Belgium, the National Solar Observatory, Stanford University, the Air Force Research Laboratory and the Specola Solare Ticinese.
Finally, the authors are also grateful to Rainer Arlt for his detailed reviewing work and constructive comments.
- R. Arlt, A. Abdolvand, First solar butterfly diagram from Schwabe’s observations in 1825–1867, in Physics of Sun and Star Spots, ed. by D.P. Choudhary, K.G. Strassmeier. IAU Symposium 273, Ventura, California, USA, August 22–26, 2010. Proc. of IAU Symposium, vol. 273 (Cambridge University Press, Cambridge, 2011), pp. 286–289 Google Scholar
- D. Berghmans, R.A.M. van der Linden, P. Vanlommel, F. Clette, E. Robbrecht, History of the Sunspot Index: 25 years SIDC. Beitrage zur Geschichte der Geophysik und Kosmischen Physik VII(1), 7, 288 (2006) Google Scholar
- W. Brunner, Letter to editor from Zurich Observatory. Terr. Magn. 41(2), 210 (1936) Google Scholar
- F. Clette, L. Lefèvre, Are the sunspots really vanishing? Anomalies in solar cycle 23 and implications for long-term models and proxies. J. Space Weather Space Clim. 2, A260000 (2012) Google Scholar
- E.W. Cliver, L. Svalgaard, in Origins of the Wolf Sunspot Number Series: Geomagnetic Underpinning, AGU Fall Meeting, Abstract SH13A-1109 (2007) Google Scholar
- M.J. Crowe, K.R. Lafortune, Herschel, (Friedrich) William [Wilhelm], in The Biographical Encyclopedia of Astronomers, ed. by T. Hockey (Springer, Berlin, 2007), pp. 494–496 Google Scholar
- W. de la Rue, B. Stewart, B. Loewy, Researches on solar physics. heliographical positions and areas of sun-spots observed with the kew photoheliograph during the years 1862 and 1863. Philos. Trans. R. Soc. Lond. 159, 1–110 (1869) Google Scholar
- T.K. Friedli, Erste beobachtungen am Wolfschen Normal refraktor in Schliern b, Köniz. Mitt. Rudolf-Wolf-Ges. 3(9), 3–46 (1997) Google Scholar
- T.K. Friedli, H.U. Keller, Rudolf Wolf als Pionier der Sonnenfleckenforschung. Vierteljahrsschr. Nat.forsch. Ges. Zür. 138(4), 267–281 (1993) Google Scholar
- G. Galilei, J.C. Scheiner, On Sunspot Translated and Introduced by, Reeves, E. and van Helden, A. (The University of Chicago Press, Chicago and London, 2010) Google Scholar
- M.N. Gnevyshev, A.I. Ohl, Astron. Zh. 25, 18 (1948) Google Scholar
- D.H. Hathaway, Royal Observatory, Greenwich—USAF/NOAA Sunspot Data (2014). http://solarscience.msfc.nasa.gov/greenwch.shtml
- J.L. Heilbron, The Sun in the Church (Harvard University Press, Cambridge, 1999), 366 p. Google Scholar
- D.V. Hoyt, K.H. Schatten, The Role of the Sun in Climate Change (Oxford University Press, New York, 1997) Google Scholar
- K. Hufbauer, Exploring the Sun: Solar Science since Galileo (Johns Hopkins University Press, Baltimore, 1991) Google Scholar
- M.J. Johnson, Address delivered by the President, M.J. Johnson, Esq. on presenting the Medal of the Society to M. Schwabe. Mon. Not. R. Astron. Soc. 17, 126–132 (1857) Google Scholar
- H. Kant, Scheiner, Christoph, in The Biographical Encyclopedia of Astronomers, ed. by T. Hockey (Springer, Berlin, 2007), p. 1018 Google Scholar
- H.U. Keller, T.K. Friedli, The sunspot-activity in the years 1976–1995. Mitt. Rudolf-Wolf-Ges. 3(7), 1–46 (1995) Google Scholar
- K.O. Kiepenheuer, Solar site testing, in Le choix des sites d’observatoires astronomiques (site testing), ed. by J. Rösch. IAU Symposium, vol. 19 (1962), pp. 193–219 Google Scholar
- J.F. Lalande, Astronomie (1771), Paris, 4 Vols. Google Scholar
- Locarno, Specola Solare Ticinese Drawings, Archive (2014). http://www.specola.ch/e/drawings.html
- M. Lockwood, M.J. Owens, L. Barnard, Centennial variations in sunspot number, open solar flux, and streamer belt width: 1. Correction of the sunspot number record since 1874. J. Geophys. Res. Space Phys. 119 (2014, in press). doi: 10.1002/2014JA019970, http://onlinelibrary.wiley.com/doi/10.1002/2014JA019970/pdf
- E. Loomis, Am. J. Sci. Ser 50, 153 (1870) Google Scholar
- E. Manfredi, De Gnomone Meridiano Bononiensi ad Divi Petronii, Laeli a Vulpa, Bononiae (1736, 397 pp.) Google Scholar
- M.J. Owens, M. Lockwood, 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, 4102 (2012) Google Scholar
- D.G. Parker, J.M. Pap, R.K. Ulrich, L.E. Floyd, D.K. Prinz, in Developing New Mount Wilson Magnetic Indices to Model Solar UV Variations, SPD meeting #28, #02.54. Bulletin of the American Astronomical Society, vol. 29 (American Astronomical Society, Washington, 1997), p. 902 Google Scholar
- M. Rybanský, V. Rušin, M. Minarovjech, L. Klocok, E.W. Cliver, Reexamination of the coronal index of solar activity. J. Geophys. Res. 110, 8106 (2005) Google Scholar
- J.W. Shirley, Thomas Harriot: A Biography (Clarendon Press, Oxford, 1983) Google Scholar
- Sonne, Fachgruppe Sonne (2014). http://www.vds-sonne.de/index.php
- G. Spörer, Vierteljahrsschr. Astron. Ges. 22, 323 (1887) Google Scholar
- L. Svalgaard, Updating the historical sunspot record, in SOHO-23: Understanding a Peculiar Solar Minimum, Proceedings of a workshop held 21–25 September 2009 in Northeast Harbor, Maine, USA, ed. by S.R. Cranmer, J.T. Hoeksema, J.L. Kohl. ASP Conference Series, vol. 428 (Astronomical Society of the Pacific, San Francisco, 2010), p. 297 Google Scholar
- L. Svalgaard, Building a Sunspot Group Number Backbone Series SSN-Workshop 3 (Tucson) (2013b). http://www.leif.org/research/SSN/Svalgaard11.pdf
- L. Svalgaard, What can geomagnetism can tell us about the solar cycle? Space Sci. Rev. (2014b, this volume) Google Scholar
- L. Svalgaard, E.W. Cliver, in Calibrating the Sunspot Number: Using “the Magnetic Needle”, AGU Spring Meeting Abstracts 5454B-02 (2007) Google Scholar
- L. Svalgaard, H.S. Hudson, The solar microwave flux and the sunspot number, in SOHO-23: Understanding a Peculiar Solar Minimum, Proceedings of a Workshop Held 21–25 September 2009 in Northeast Harbor, Maine, USA, ed. by S.R. Cranmer, J.T. Hoeksema, J.L. Kohl. ASP Conference Series, vol. 428 (Astronomical Society of the Pacific, San Francisco, 2010), p. 325 Google Scholar
- J.M. Vaquero, M. Vázquez, The Sun Recorded Through History. Astrophysics and Space Science Library, vol. 361 (Springer, Berlin, 2009), 382 p. Google Scholar
- M. Waldmeier, Astron. Mitteil. Eidgn. Sternw. Zürich, No. 152 (1948) Google Scholar
- M. Waldmeier, The sunspot-activity in the years 1610–1960 (Schulthess & Co., Swiss Federal Observatory, Zürich, 1961) Google Scholar
- M. Waldmeier, Die Beziehung zwischen der Sonnenflecken-relativ-zahl und der Gruppenzahl. Astr. Mitteil. Eidgn. Sternw Zürich No. 285 (1968) Google Scholar
- R.M. Wilson, D.H. Hathaway, On the Relation Between Sunspot Area and Sunspot Number. NASA STI/Recon Technical Report N, 6, 20186 (2006) Google Scholar
- R. Wolf, Sonnenflecken Beobachtungen in der zweiten Hälfte des Jahres 1850. Mitt. Nat.forsch. Ges. Bern 207, 89–95 (1851) Google Scholar