= Solar cycle
The solar cycle is the recurrent variation of magnetic activity, with a characteristic sunspot-number period of about eleven years. The large-scale magnetic polarity reverses from one activity cycle to the next, giving an approximately twenty-two-year magnetic cycle. Activity cycles vary in length and amplitude. A satisfactory <solar dynamo> model must account for polarity reversal, equatorward activity-belt migration in the <solar butterfly diagram>, the phase relation between polar and active-region fields, and long-term modulation including <solar grand minima>.
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