The New Sunspot-Number Index and Solar-Cycle Characteristics

Abstract

We revisit several characteristics of the solar cycle using the new version of the sunspot-number index. Thus, we calculated several correlations, including the recent Solar Cycles 23 and 24 in the analysis. We applied two smoothing methods to the sunspot number: i) the usual 13-month running mean and ii) a 24-month Gaussian filter. Each of these methods contains two analyses: on the one hand, we consider all of the solar cycles available, and on the other hand, only those from Solar Cycle 10 onward. It can be seen that this new version improves or yields similar results for the correlations with respect to other works using the old version of the sunspot number, except for the amplitude–descending time effect and the linear fit of the secular trend. However, employing the same methodology in the analysis and considering the same solar cycles, it can be seen that the new sunspot number, in general, does not improve the correlations with respect to the old sunspot number and, moreover, the correlations obtained with the Gaussian filter generally are stronger than those with the 13-month running mean. Furthermore, from a sinusoidal fit to the solar-maximum amplitudes of the whole series, we have obtained a periodicity of the Gleissberg cycle equal to 97.7 years ( ${\approx}\,8.9$ solar cycles) for the 13-month running mean and 99.8 years ( ${\approx}\,9.1$ solar cycles) for the Gaussian filter. Lastly, the Waldmeier effect, the modified Waldmeier effect, the amplitude–period effect, the amplitude–minimum effect, and the even–odd effect are characteristics with high correlation coefficients and significance levels; the sinusoidal fit applied to the solar-maximum amplitudes yields a lower correlation coefficient value but a high significance level; and both the amplitude–descending-time effect and secular trend of the solar activity have weaker correlation coefficients and significance levels.