Analysing Spotless Days as Predictors of Solar Activity from the New Sunspot Number

Abstract

The use of spotless days to predict future solar activity is revised here based on the new version of the sunspot number index with a 24-month filter. Data from Solar Cycle (SC) 10 are considered because the temporal coverage of the records is 100% for this solar cycle. The interrelationships of the timing characteristics of spotless days and their comparison with sunspot cycle parameters are explored; in some cases, we find very strong correlations. Such is the case for the relationship between the minimum time between spotless days either side of a given solar maximum and the maximum time between spotless days either side of the previous solar minimum, with $r = -0.91$ and a $p\mbox{-value} < 0.001$. However, the predictions for SCs 24 or 23 made by other authors in previous works using spotless days as a predictor of solar activity are not correct since the predictions have not been fulfilled. Although there seems to be a pattern of strong correlation for some relationships between the parameters that have been studied, a prediction of future solar cycles from these parameters defined as functions of spotless days should be made with caution because the estimated values are sometimes far from the observed ones. Furthermore, SC 23 seems to show a mode change, a break with respect to the behaviour of previous solar cycles and more similar to SCs 10 – 15.