Changes in solar irradiance are reflected by the number of sunspots that recent satellite measurements confirm. The spot number, which is accurately known since the 17th century, shows that solar irradiance varies mainly in four bands. They are the Schwabe cycle of 9-13 years, the Jupiter – Saturn cycle of 59 years, the Gleissberg cycle of 90-110 years and the DeVries and Suess cycle of 180-220 years. Cosmogenic isotopes[i] confirm the existence of these and other longer solar cycles such as the Hallstattzeit cycle of 2,300 years.
The 59-year cycle results from the shifting of the center of gravity of the solar system relative to the center of the sun, in relation to periods of rotation of the largest planets, i.e. Jupiter and Saturn. The Gleissberg cycle, which was discovered in 1958, possibly is a modulation of the amplitude of the 11-year Schwabe cycle, which itself results from the differential rotation of the convection zone of the sun, periodically inducing a deformation followed by the consolidation of the magnetic flux tubes. The origin of the cycles of longer periods is not known with certainty.
[i] Cosmogenic isotopes are produced by cosmic rays in the upper atmosphere, forming rare isotopes of hydrogen (3H) and produces helium (3He), beryllium (10Be), carbon (14C), neon (21Ne), aluminum (26Al), and chlorine (36Cl). Isotopic abundance reflects solar activity: during periods of high activity cosmic rays are deflected out the solar system and therefore produce less cosmogenic isotopes.