The variation in the number of sunspots and other forms of solar activity with an average period of about 11 years, although the actual lengths have varied from 9 to 14 years. Normally the rise to maximum is quicker than the subsequent decline. Some cycles exhibit a double peak, due to activity in each hemisphere reaching a maximum at different times. In each successive cycle the north and south magnetic polarities of the Sun are reversed, so that there is a 22-year magnetic cycle. The monthly averages of relative sunspot numbers are around 6 in years of sunspot minimum and 116 at sunspot maximum. Not only the sunspot number varies, but also the numbers of active regions and flares, and the level of ultraviolet, radio, and other emission associated with such regions. The 11-year periodicity is thought to arise through the action of the solar dynamo. The sunspot cycle may not always have operated in its present observed form, for example at the time of the Maunder minimum.