Abstract/Summary

We use five test data series to search for, and quantify, putative discontinuities around 1946 in five different annual-mean sunspot-number or sunspot-group number data sequences. The data series tested are: the original and new versions of the Wolf/Zurich/International sunspot number composite [RISNv1 and RISNv2] (respectively Clette et al., Adv. Space Res., 40, 919, 2007 and Clette et al., in “The Solar Activity Cycle”, 35, Springer, 2015); the corrected version of RISNv1 proposed by Lockwood, Owens, and Barnard (J. Geophys. Res. Space Physics, 119, 5193, 2014a) [RC]; the new “backbone” group number composite proposed by Svalgaard and Schatten (Solar Physics, 2016) [RBB]; and the new group-number composite derived by Usoskin et al. (Solar Physics, 2016) [RUEA]. The test data series used are: the group number [NG] and total sunspot area [AG] from the Royal Observatory, Greenwich / Royal Greenwich Observatory (RGO) photoheliographic data; the Ca K index from the recent re-analysis of Mount Wilson Observatory (MWO) spectroheliograms in the Calcium II K ion line; the sunspot-group number from the MWO sunspot drawings [NMWO]; and the dayside ionospheric F2-region critical frequencies measured by the Slough ionosonde [foF2]. These test data all vary in close association with sunspot numbers, in some cases non-linearly. The tests are carried out using both the “before-and-after” fit-residual comparison method and the correlation method of Lockwood, Owens, and Barnard, applied to annual mean data for intervals iterated to minimise errors and to eliminate uncertainties associated with the precise date of the putative discontinuity. It is not assumed that the correction required is by a constant factor, nor even linear in sunspot number. It is shown that a non-linear correction is required by RC, RBB, and RISNv1, but not by RISNv2 or RUEA. The five test datasets give very similar results in all cases. By multiplying the probability distribution functions together we obtain the optimum correction for each sunspot dataset that must be applied to pre-discontinuity data to make them consistent with the post-discontinuity data. It is shown that, on average, values for 1932 - 1943 are too small (relative to later values) by about 12.3 % for RISNv1 but are too large for RISNv2 and RBB by 3.8 % and 5.2 %, respectively. The correction that was applied to generate RC from RISNv1 reduces this average factor to 0.5 % but does not remove the non-linear variation with the test data, and other errors remain uncorrected. A valuable test of the procedures used is provided by RUEA, which is identical to the RGO NG values over the interval employed.