Sunspot numbers and spotless days around solar minimum

In this work, we have statistically explored the relationship between solar activity and the occurrence of both spotless days and active days. The analysis is performed using the latest version of the daily sunspot number data compiled by the Sunspot Index and Long-Term Solar Observations, covering the period from January 1818 to June 2025, spanning solar cycles 6–25. We have calculated linear correlation coefficients for various combinations of the numbers of spotless days and sunspot numbers, determined in different ways. The results show that the mean sunspot number for a solar cycle is more significantly anti-correlated with the number of spotless days compared with the yearly mean sunspot numbers at the solar maximum. Noting the feature that a weak solar cycle is typically considered to be led by a solar minimum with a large number of spotless days, solar cycles 16, 20, 21, 22, and 23 among strong ones appear somewhat weaker than expected based on the very small number of spotless days. Nonetheless, it turns out that the skewed distribution of monthly spotless days (%) from the cycles identified to be weaker than expected is closer to that from other strong cycles than from weak ones. It is also found that the occurrence rate of sunspots on an active day during solar cycles 16, 20, 21, 22, and 23 is suggested to be lower than that for other cycles.