The solar cycle periodically reshapes the magnetic structure and radiative output of the Sun and determines its impact on the heliosphere roughly every 11 years. Besides this main periodicity, it shows century-long variations (including periods of abnormally low solar activity called grand minima). The Maunder Minimum (1645–1715) has generated significant interest as the archetype of a grand minimum in magnetic activity for the Sun and other stars, suggesting a potential link between the Sun and changes in terrestrial climate. Recent reanalyses of sunspot observations have yielded a conflicted view on the evolution of solar activity during the past 400 years (a steady increase versus a constant level). This has ignited a concerted community-wide effort to understand the depth of the Maunder Minimum and the subsequent secular evolution of solar activity. The goal of this Perspective is to review recent work that uses historical data to estimate long-term solar variability, and to provide context to users of these estimates that may not be aware of their limitations. We propose a clear visual guide than can be used to easily assess observational coverage for different periods, as well as the level of disagreement between currently proposed sunspot group number series.
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A Python Jupyter notebook, along with the necessary data to reproduce all plots presented here can be found at https://github.com/amunozj/NatAs_SN_Perspective. We request that you use data in this repository only to reproduce our analysis, but get them from the original sources if you intend to use them for research purposes.
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We thank the International Space Science Institute (ISSI-Bern) and the members of its team for the Recalibration of the Sunspot Number Series for providing the support and insight that made this Perspective possible. We also thank the ISSI team and L. Svalgard for useful comments and suggestions. This research is partly funded by the NASA Grand Challenge grant NNX14AO83G and NASA LWS grant NNX16AB77G, by the Economy and Infrastructure Counselling of the Junta of Extremadura through project IB16127 and grant GR15137 (co-financed by the European Regional Development Fund), and by the Ministerio de Economía y Competitividad of the Spanish Government (AYA2014-57556-P and CGL2017-87917-P). Sunspot number series data are provided by the World Data Center SILSO, Royal Observatory of Belgium, Brussels, or kindly provided by the original authors10,18,21,23. Butterfly diagram data are provided by the St. Petersburg State University41, the Historical Archive of Sunspot Observations (HASO) of the Universidad de Extremadura34,36, the Leibniz Institute for Astrophysics Potsdam (AIP)30,33,35,38, the Debrecen Observatory37,39 and the Kislovodsk Astronomical Mountain Station40.