Tests of Sunspot Number Sequences: 4. Discontinuities Around 1946 in Various Sunspot Number and Sunspot-Group-Number Reconstructions

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 [R-ISNv1 and R-ISNv2] (respectively Clette et al. in Adv. Space Res. 40, 919, 2007 and Clette et al. in The Solar Activity Cycle 35, Springer, New York, 2015); the corrected version of R-ISNv1 proposed by Lockwood, Owens, and Barnard (J. Geophys. Res. 119, 5193, 2014a) [R-C]; the new "backbone" group-number composite proposed by Svalgaard and Schatten (Solar Phys. 291, 2016) [R-BB]; and the new group-number composite derived by Usoskin et al. (Solar Phys. 291, 2016) [R-UEA]. The test data series used are the group-number [N-G] and total sunspot area [A(G)] from the Royal Observatory, Greenwich/Royal Greenwich Observatory (RGO) photoheliographic data; the CaK 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 [N-MWO]; 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 R-C, R-BB, and R-ISNv1, but not by R-ISNv2 or R-UEA. 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 low (relative to later values) by about 12.3 % for R-ISNv1 but are too high for R-ISNv2 and R-BB by 3.8 % and 5.2 %, respectively. The correction that was applied to generate R-C from R-ISNv1 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 R-UEA, which is identical to the RGO N-G values over the interval employed.