The Phase of the Solar Cycle
The solar cycle is commonly regarded as having an 11 year period. This is true but only as an average value. Actual cycles have a period as short as 9 years or as long as 14 years. Moreover, there are substantial intervals of time during which the cycle is consistently shorter than 11 years and other intervals during which the period is consistently longer than 11 years.
The figure shows the occurrence of solar minimum compared with an average 11 year period (red line). Prior to cycle 4 (commenced 1784) cycles were generally shorter than 11 years. From that time until solar cycle 15 (commenced 1913) the period was mostly longer than 11 years. Since then mostly cycles have been shorter than 11 years.
As years progress and further data comes to hand we may begin to see a periodicity in the length of cycles. What do the next few cycles have in store?
Material prepared by Richard Thompson