The Length of the Solar Cycle
Most people think of the solar cycle as having a fixed length of 11 years. This is not strictly true as cycles vary considerably in length from as little as 9 years up to almost 14 years.
The figure is a histogram showing the length of cycles, in bins of 0.5 years, for Cycle 1 (peak in 1761) up to the most recently completed one, Cycle 21 (peak in 1979). We have records of earlier cycles but these are generally regarded as not being reliable and so have not been included in the diagram.
The figure demonstrates that there is no single value for the cycle length with values spread across the graph. The most common cycle length is around 10.5 years and there are also quite a few cycles in the range 11.5-12.5 years. Interestingly, only one cycle falls close to the average cycle length of 11.1 years. This leads some people to believe that solar cycles typically come in two varieties with long and short periods. The long ones have a period of around 12 years and the short ones a period of around 10 years. In this view, the traditional cycle length of 11 years is just the average of these two types and is not representative of either variety!
The current solar Cycle 22 is declining rapidly from its peak in 1989 and the low point of the cycle (called solar minimum) should be reached in 3-4 years. What will be the length of Cycle 22? With a length that is the same of the shortest of these 21 solar cycles, we could expect solar minimum in late 1995. On the other hand, if the cycle length is the same as the longest, then minimum will be delayed until the middle of the year 2000. If the cycle has an average length of 11.1 years then we can expect solar minimum in late 1997. Yet another possible view is that the cycle will have the same length as most cycles this century (around 10.3 years) and solar minimum will occur in late 1996.
There are certainly grounds for a wide range of views about the timing of the next solar minimum and, not surprisingly, there is no consensus even amongst the experts. Ultimately, we will have to wait some years until the issue is resolved and Cycle 23 is underway.
Material prepared by Richard Thompson