Solar Activity Cycle

The solar activity cycle is the periodic, typically 11-year-long variation in the number of active features (for example, sunspots) visible on the Sun's apparent surface or in its atmosphere. Over a period of 11 years, the number of sunspots gradually rises from a low level, reaches a maximum near the midpoint of the cycle, and then declines to a minimum. Solar activity is governed by the sun's magnetic field, and one of the unsolved problems in astronomy is the origin of the regular changes in the magnetic field that drive the activity cycle.