Number of Spotless Days in Relation to the Timing and Size of Sunspot Cycle Minimum

Abstract

Sunspot cycle (SC) 24, the present ongoing SC, is now in its 9th year, having had a minimum in annual sunspot number (SSN) in 2008, measuring 4.2, and a minimum in ‘smoothed’ SSN in December 2008, measuring 2.2. Its maximum annual SSN occurred in 2014, measuring 113.3, and its maximum smoothed SSN occurred in April 2014, measuring 116.4. Following smoothed SSN maximum (SSN max), the first spotless day (FSD) during the decline of SC24 was reported in July 2014, a mere 3 months following SC24’s maximum SSN amplitude, an interval shorter than the 13 months found for SC12 and the 8 months for SC14. Through December 2016, some 27 spotless days (1 in 2014 and 26 in 2016) have been seen, with more to come as SC25’s epoch of minimum approaches. Examined in this study are the following three factors: (1) the variation in number of spotless days (NSD) relative to the epochs of sunspot minimum (SSN min) and SSN max, (2) the association between the maximum NSD and the SSN min, and (3) cyclic variations of selected spotless day-associated parameters. It is suggested that SC25’s epoch of SSN min likely will occur sometime in 2020 or later and that SC24’s NSD behavior represents a return to that experienced during SC12 and SC14. Hence, another prolonged minimum, like that experienced for SC23/24, might well occur for SC24/25. For comparison, SC24’s interval of spotless days from first to last spotless day bounding SSN min extended 84 months and totaled 816 spotless days, with 265 spotless days reported during the year of annual SSN min.