Abstract
Four new sunspot number time series have been published in this Topical Issue: a backbone-based group number in Svalgaard and Schatten (Solar Phys., 2016; referred to here as \(\mathit{SS}\), 1610 – present), a group number series in Usoskin et al. (Solar Phys., 2016; UEA, 1749 – present) that employs active day fractions from which it derives an observational threshold in group spot area as a measure of observer merit, a provisional group number series in Cliver and Ling (Solar Phys., 2016; \(\mathit{CL}\), 1841 – 1976) that removed flaws in the Hoyt and Schatten (Solar Phys. 179, 189, 1998a; 181, 491, 1998b) normalization scheme for the original relative group sunspot number (\(R_{\mathrm{G}}\), 1610 – 1995), and a corrected Wolf (international, \(R_{\mathrm{I}}\)) number in Clette and Lefèvre (Solar Phys., 2016; \(S_{\mathrm{N}}\), 1700 – present). Despite quite different construction methods, the four new series agree well after about 1900. Before 1900, however, the UEA time series is lower than \(\mathit{SS}\), \(\mathit{CL}\), and \(S_{\mathrm{N}}\), particularly so before about 1885. Overall, the UEA series most closely resembles the original \(R_{\mathrm{G}}\) series. Comparison of the UEA and SS series with a new solar wind \(B\) time series (Owens et al. in J. Geophys. Res., 2016; 1845 – present) indicates that the UEA time series is too low before 1900. We point out incongruities in the Usoskin et al. (Solar Phys., 2016) observer normalization scheme and present evidence that this method under-estimates group counts before 1900. In general, a correction factor time series, obtained by dividing an annual group count series by the corresponding yearly averages of raw group counts for all observers, can be used to assess the reliability of new sunspot number reconstructions.
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Notes
The average number of observers per year in the Hoyt and Schatten data base for decades after 1850 is as follows: 1850s (7), 1860s (11), 1870s (12), 1880s (15), 1890s (17), 1900s (23), 1910s (19), 1920s (9), and 8 or more through the 1980s (excluding Mt. Wilson, center of disk).
These \(k\)-factors are based on direct comparison to the RGO except for Schwabe and Shea, who were scaled to Schmidt after that observer had been scaled to the RGO.
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Acknowledgements
I thank Leif Svalgaard and Frédéric Clette for comments on the manuscript and helpful discussions generally and acknowledge a debt of gratitude to all of the participants in the Sunspot Number Workshops. I thank Ilya Usoskin for helpful comments on the manuscript.
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Sunspot Number Recalibration
Guest Editors: F. Clette, E.W. Cliver, L. Lefèvre, J.M. Vaquero, and L. Svalgaard
Appendix: Solar Activity Time Series
Appendix: Solar Activity Time Series
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Cliver, E.W. Comparison of New and Old Sunspot Number Time Series. Sol Phys 291, 2891–2916 (2016). https://doi.org/10.1007/s11207-016-0929-7
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DOI: https://doi.org/10.1007/s11207-016-0929-7