Revisiting the Sunspot Number
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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.
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