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 (diamond symbols) compared with a regular 11 year period. Whenever the cycle is typically longer (from one minimum to the next solar minimum) than 11 years an upwards trend will be seen in the figure. On the other hand, if the cycle is shorter than 11 years a downwards trend will be seen.

The figure shows that prior to solar cycle number 3 (minimum in 1775) the period of the cycle was generally shorter than 11 years. From that time until solar cycle 15 (minimum in 1913) the period was mostly longer than 11 years. Since then, solar cycles have again been shorter than 11 years. Approximate trends are shown in the figure as a series of dashed lines, although it would also be reasonable to fit a curved line through the data.

The "star" symbol on the right hand side of the figure is based on the assumption that the solar minimum at the end of cycle 22 will occur in mid 1996, making cycle 22 a little less than 10 years in length. A cycle of this length, although much shorter than 11 years, is fairly typical of the cycles seen since 1913 and continues the trend of cycles shorter than 11 years.

Material Prepared by Richard Thompson. © Copyright IPS - Radio and Space Services.