Comparison of New and Old Sunspot Number Time Series

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 Lefevre (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.