Simple methods for predicting the size and timing of sunspot cycle 25

  • Robert M. Wilson NASA Marshall Space Flight Center, NSSTC, Huntsville, Alabama


This paper describes several simple methods for estimating the main features of an ongoing solar cycle (SC); in particular, its maximum amplitude (RM), ascent duration (ASC), and period (PER). The current ongoing SC25 had its minimum amplitude (Rm) in December 2019, measuring 1.8. At elapsed time t = 24 months (December 2021), smoothed monthly mean sunspot number R measured 55.7. Comparisons of R with the mean values of Fast–rising–Slow–rising and Short–PER–Long–PER cycles strongly suggest that SC25 is best described as being a Slow–rising–Long–PER SC (i.e., Slow–Long), inferring an ASC ≥49 months (i.e., RM occurrence on or after January 2024) and PER ≥131 months (i.e., Rm occurrence for SC26 on or after November 2030). Furthermore, RM >116.4 is expected for SC25 based on the inferred Even–Odd preferential relationship (i.e., during the modern era of sunspot observations SC12–SC24, the Odd–following SC in consecutive Even–Odd cycle pairs has usually had the larger RM, true for 5 of 6 Even–Odd cycle pairs). Slow–Long cycles (i.e., SCs 12, 14, 20, 23, and 24) have an average RM = 132.0 ± 30.6, ASC = 57 ± 7 months and PER = 138 ± 6 months. Based on the overlap of the geomagnetic minimums (i.e., Aam and Apm) in the vicinity of Rm, one estimates RM = 145.7 ± 17.1 for SC25, a value in stark contrast to the panel prediction of RM = 115 peaking in July 2025 ± 8 months. The greatest rate of growth (GRG) in R observed thus far for SC25 is only 5.6 occurring at t = 22 months (October 2021); which, if it holds up, would be the smallest GRG during the modern era (SC12–SC24) and would suggest an RM = 107.6 ± 33.4 and a mean rate of growth (MRG) = 1.7 ± 0.9 for SC25. For comparison, the smallest GRG during SC12–SC24 is 6.5 occurring at t = 39 months associated with SC14, a Slow–Long SC, having RM = 107.1, ASC = 49 months, MRG = 2.09, and PER = 138 months; SC24 had RM = 116.4, ASC = 64 months, GRG = 8.2 at t = 28 months, MRG = 1.78, and PER = 132 months.