The solar cycle is a recurrent pattern of magnetic activity in the Sun. It was first identified in 1843 by the German observer Samuel Heinrich Schwabe from the approximately 11-year cycle in the number of sunspots seen on the solar disk. Sunspots are among the observable features of localized magnetic disturbances in the magnetically active solar atmosphere. Their cycle corresponds to similar 11-year cycles of other such features in these active regions, such as the number of faculae, the rate of incidence of solar flares, and the intensity of coronal X-ray and radio-frequency emissions. Two consecutive sunspot cycles complete a 22-year solar magnetic cycle, in which both the number and the magnetic polarities of the bipolar spot groups return to their initial values.

The solar cycle has been relatively regular for many decades. Records extending to the 17th century, however, show that its amplitude may vary dramatically over longer time scales.

Peter V. Foukal