摘要：Acknowledgments We acknowledge support from the Royal Observatory of Belgium, the Belgian Solar-Terrestrial Center of Excellence, the National Solar Observatory, the U.S. Air Force Research Laboratory, Stanford University, Universidad de Extremadura, Specola Solare Ticinese, and Istituto Ricerche Solari Locarno. Data on which Figure 1a is based can be found at http://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-indices/sunspot-numbers/group/ and http://www.sidc.be/silso/versionarchive.
