[INTRODUCTION]      [PRECURSORS]
SOLAR CYCLE

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 number	Minimum date	Minimum RI12	Maximum date	Maximum RI12	Increase duration	Decrease duration	Total duration
1	03/1755	14.0	06/1761	144.1	75	60	11y 3m
2	06/1766	18.6	09/1769	193.0	39	69	9y 0m
3	06/1775	12.0	05/1778	264.3	35	76	9y 3m
4	09/1784	15.9	02/1788	235.3	41	122	13y 7m
5	04/1798	5.3	02/1805	82.0	82	65	12y 3m
6	07/1810	0.0	05/1816	81.2	70	84	12y 10m
7	05/1823	0.1	11/1829	119.2	78	48	10y 6m
8	11/1833	12.2	03/1837	244.9	40	76	9y 8m
9	07/1843	17.6	02/1848	219.9	55	94	12y 5m
10	12/1855	0.1	02/1860	186.2	50	85	11y 3m
11	03/1867	9.9	08/1870	234.0	41	100	11y 9m
12	12/1878	3.7	12/1883	124.4	60	75	11y 3m
13	03/1890	8.3	01/1894	146.5	46	96	11y 10m
14	01/1902	4.5	02/1906	107.6	49	89	11y 6m
15	07/1913	2.5	08/1917	175.7	49	71	10y 0m
16	07/1923	9.4	04/1928	130.2	57	65	10y 2m
17	09/1933	5.8	04/1937	198.6	43	82	10y 5m
18	02/1944	12.9	05/1947	218.7	39	83	10y 2m
19	04/1954	5.1	03/1958	285.0	47	79	10y 6m
20	10/1964	14.3	11/1968	156.6	49	91	11y 8m
21	06/1976	17.9	12/1979	232.9	42	81	10y 3m
22	09/1986	13.5	07/1989	232.9	34	82	9y 8m
23	05/1996	11.2	04/2000	175.2	47	104	12y 7m
24	12/2008	2.2	04/2014	116.4	64	68	11y 0m
25	12/2019	1.8
Mean		9.11		179.36	51.3m (4y 3.3m)	81m (6y 9m)	11y 0.3m
Median		9.65		180.95	48m (4y)	81.5m (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: