solar cycle

The periodic variation in the amount of solar activity, particularly the number of sunspots. The period of the cycle is about 11 years, though it has been closer to 10 years during the twentieth century. At the commencement of a new cycle there are few, if any, spots on the Sun. The first ones for the new cycle erupt around heliographic latitudes 35°-45° north and south; over the course of the cycle, subsequent spots appear closer to the equator, finishing at around 7° north and south. This pattern can be demonstrated graphically as a butterfly diagram. It is generally thought that the solar cycle is caused by an interaction between the "dynamo" responsible for the Sun's magnetism and the Sun's rotation. The Sun does not rotate as a solid body: the equatorial regions rotate fastest and this amplifies the magnetic field, which eventually bursts into the photosphere, causing sunspots. At the end of each cycle the overall magnetic field reverses, giving a total period of 22 years, which is known as the Hale cycle.