Analysing the spotless days as predictors of the solar activity from the new sunspot number

TL;DR

This study revises the use of spotless days to predict solar activity using the new sunspot number index, revealing strong correlations but also highlighting limitations in predictive accuracy for recent cycles.

Contribution

It introduces an analysis of the relationship between spotless days and solar cycle parameters using the updated sunspot number, emphasizing caution in predictions based on these parameters.

Findings

Strong correlation between certain spotless day parameters and solar cycle features.

Previous predictions based on spotless days did not accurately forecast recent cycles.

SC 23 shows a mode change, resembling earlier cycles rather than recent ones.

Abstract

The use of the spotless days to predict the future solar activity is here revised based on the new version of the sunspot number index with a 24-month filter. Data from Solar Cycle (SC) 10 are considered because from this solar cycle the temporal coverage of the records is 100 %. The interrelationships of the timing characteristics of spotless days and their comparison with sunspot cycle parameters are explored, in some cases finding 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 in the previous solar minimum, with r = -0.91 and a p-value < 0.001. However, the predictions for SC 24 or 23 made by other authors in previous works using the spotless days as a predictor of solar activity are not good since it has not been fulfilled. Although there seems to be a pattern of strong correlation for some relationships between the parameters studied, prediction of future solar cycles from these parameters defined as functions of the spotless days should be made with caution because sometimes the estimated values are far from the observed ones. Finally, SC 23 seems to show a mode change, a break respect to the behavior of their previous solar cycles and more similar to SC 10-15.