Veteran solar researcher Timo Niroma has an elegant and simple analysis on his main sunspots page which neatly shows the bi-modal nature of the solar cycle lengths using ascii art! It’s common knowledge that the average solar cycle length is just over 11 years. What isn’t so well known is that the actual solar cycle length clusters around two different periods of around 10.38 and 12 years.
In the next table I have drawn lengths of the cycles so that the “official” value gets four points, the nearest value three points, the tenths of years whose distance is 0.2 years get two points and finally one point is given at the distance of 0.3 years. This should compensate for the inaccuracy of the values. For the years 5 and 6 I have used the calibrated values, for the cycle 22 the traditional value. The tentative cycle 0 (10.2 years) is added with a “o” notation.
This Bimodal distribution has been noted by other researchers too. Ray Tomes says:
A good alignment is one where the Earth has a near zero misalignment with J-V. Starting from an assumed perfect alignment, the second J-V period gives a moderate alignment, but the fifth gives a good alignment after an interval of 3.244 years. Multiples of 5 J-V periods thereafter get progressively worse until it becomes necessary to add an extra 2 J-V periods and the alignments then get better every 5 periods. These correspond to periods of 20.76 and 24.00 years. For some unknown reason, the gravitational oscillations reach maxima at intervals of 10.38 and 12.00 years, whereas best alignments take twice as long.
I’ll keep this post short and sweet, as it gets complicated next. I’d just like the bi-modal nature of solar activity to sink in first.