The sun is a star which magnetic field varies with time. The sunspots, which are regions with strong magnetic fields, provide an easy way to follow the 11-year solar activity cycle variations. The sunspots and their groups are counted in many observatories and an index, called sunspot number (or more properly sunspot index) is derived by the Royal Observatory of Belgium SILSO service from the collected observations. The cycle duration varies from 9 to 14 years, 11 years being the average as shown in the following tables which gives values of the RI12 index(*).

Almost all the solar features are directly or indirectly related to the solar cycle. Indeed the solar magnetic field structures the entire solar atmosphere and, as a consequence, the interplanetary medium. Thus the solar cycle also modulates number of solar-terrestrial effects. Prediction of the cycle of sunspot numbers are of prime importance for most of the Space Weather applications. It is strongly needed for telecommunications and satellite orbitography because the solar flux in the UV, EUV and X-ray ranges, which is partly emitted in magnetic loops located above solar active regions, is highly correlated to the sunspot number. Radiation belts characteristics (and their impact on satellite electronics) are also solar cycle dependent, as well as atmospheric chemistry (and its impact on satellite surfaces). Because solar flare and coronal mass ejection frequencies are closely related to the sunspot cycle, applications involving solar particles (radiation dose received by astronauts and by satellite electronics for example) are demanding long term sunspot cycle predictions. Finally solar sunspot cycle prediction is used to predict cosmic ray intensity and radiation doses received by air crews, because of the galactic cosmic ray modulation by heliospheric magnetic field (see modulation parameters).

To learn more about the solar cycle, you can read the following chapter:   (pdf, 314 Ko, in french)

Cycle numberMinimum dateMinimum RI12Maximum dateMaximum RI12Increase durationDecrease durationTotal duration
103/175514.006/1761144.1756011y 3m
206/176618.609/1769193.039699y 0m
306/177512.005/1778264.335769y 3m
409/178415.902/1788235.34112213y 7m
504/17985.302/180582.0826512y 3m
607/18100.005/181681.2708412y 10m
705/18230.111/1829119.2784810y 6m
811/183312.203/1837244.940769y 8m
907/184317.602/1848219.9559412y 5m
1012/18550.102/1860186.2508511y 3m
1103/18679.908/1870234.04110011y 9m
1212/18783.712/1883124.4607511y 3m
1303/18908.301/1894146.5469611y 10m
1401/19024.502/1906107.6498911y 6m
1507/19132.508/1917175.7497110y 0m
1607/19239.404/1928130.2576510y 2m
1709/19335.804/1937198.6438210y 5m
1802/194412.905/1947218.7398310y 2m
1904/19545.103/1958285.0477910y 6m
2010/196414.311/1968156.6499111y 8m
2106/197617.912/1979232.9428110y 3m
2209/198613.507/1989232.934829y 8m
2305/199611.204/2000175.24710412y 7m
2412/20082.204/2014116.4646811y 0m
Mean 9.11 179.3651.3m
(4y 3.3m)
(6y 9m)
11y 0.3m
Median 9.65 180.9548m
(6y 9.5m)
10y 9.5m

(*) In july 2015 the SIDC changed the way the RI12 index is computed (more details on SILSO website). As a result, minimum and maximum values of current and past cycles where shifted to significantly higher values (cycle duration average remains almost unchanged). Previous values of the RI12 can be found in the following table: