The 11 year cycle can be seen to vary in its amplitude and length (in the range from 9-14 years). The longer cycles tend too be associated with and followed by cycles of smaller amplitude.
Cycle 23 which peaked in 2000 is in its dying stages. Cycle 24 has yet to assert itself, although there were a few months back a few small spots with cycle 24 polarity (the polarity reverses across the solar equator and from one cycle to the next (why there is a 22 year cycle). A cycle 24 spot cluster was observed in September but it lasted just 24 hours.
Note that cluster of zero month years in the early 1800s (a very cold period called the Dalton minimum - at the time of Charles Dickens and snowy London town and including thanks to the major volcanic eruption of Tambora, the Year without a Summer 1816), and again to a lesser degree in the early 1900s. These correspond to the 106 and 213 year cycle minimums.
There was a stretch of 21 straight months from October 1809 to May 1811 without a single observed spot. Two other long periods included 4 months from November 1822 to February 1823 and 4 months from August to November of 1823.
The superimposition of the 106 and 213 year cycles would suggest that the next cycle minimum around 2020 could be especially weak. Even David Hathaway of NASA who has been a believer in the cycle 24 peak being strong, thinks the next minimum and cycle 25 maximum could be the weakest in centuries based on slowdown of the plasma conveyor belt on the sun.
"Normally, the conveyor belt moves about 1 meter per second—walking pace," says Hathaway. "That's how it has been since the late 19th century." In recent years, however, the belt has decelerated to 0.75 m/s in the north and 0.35 m/s in the south. "We've never seen speeds so low."
"The slowdown we see now means that Solar Cycle 25, peaking around the year 2022, could be one of the weakest in centuries," says Hathaway.
In this plot of the cycle lengths and sunspot number at peak of the cycles, assuming this upcoming cycle will begin in 2009 show the similarity of the recent cycles to cycle numbers 2- 4, two centuries ago preceding the Dalton Minimum. This cycle 23 could end up the longest since cycle 4, had a similar sunspot peak and similarly had two prior short cycles.
Note how this cycle has compared with the prior four cycles. Three by the 12th year were well on their way back to high activity levels. Only cycle 20, was a long and quiet cycle but its minimum was not as low and it was starting to recover by this point in time.
In a very interesting paper presented here, Livingston and Penn of the National Solar Observatory in Tucson observed “Sunspots are cool dark regions on the solar surface with strong magnetic fields. There have been few direct measurements of changes in the physical parameters of sunspots, but here we present a study which shows that sunspots are becoming warmer and have weaker magnetic fields. The number of sunspots visible on the Sun normally shows an 11-year periodicity, and the current sunspot cycle (cycle 23) had a maximum in 2001, and is entering a minimum phase with few sunspots currently visible.
Our data show that there are additional changes occurring in sunspots, independent of the sunspot cycle, and these trends suggest that sunspots will disappear completely. Such an event would not be unprecedented, since during a famous episode from 1645-1715, known as the Maunder Minimum, the normal 11-year periodicity vanished and there were virtually no sunspots visible on the solar surface (Eddy 1976). Recent studies of the appearance rate and latitudinal drift of sunspots (Hathaway et al., 2004) and of the solar magnetic field (Svalgaard etal, 2005) predict that the number of sunspots visible in future cycles will be significantly reduced. Finally the occurrence of prolonged periods with no sunspots is important to climate studies, since the Maunder Minimum was shown to correspond with the reduced average global temperatures on the Earth (Foukal et al., 1990).”
The graph shows the line depth of OH 1565.3 nm for individual spots. The upper trace is the smoothed sunspot number showing the past and current sunspot cycles; the OH line depth change seems to smoothly decrease independently of the sunspot cycle.