Sunspot cycle

From Glossary of Meteorology

sunspot cycle

A cycle with an approximate length of 11 years, but varying between about 7 and 17 years, in the number and area of sunspots, as given by the relative sunspot number.

This number rises from a minimum of 0–10 to a maximum of 50–140 in about four years time, and then declines more slowly. An approximate 11-year cycle has been found or suggested in geomagnetism, frequency of aurora, and other ionospheric characteristics. The u index of geomagnetic intensity variation shows one of the strongest known correlations to solar activity.
See also solar cycle.