The solar cycle is a recurrent pattern of magnetic activity in the
It was first identified in 1843 by the German observer Samuel Heinrich
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
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.