The sunspot cycle

Unlike the energy output of the Sun, the number of sunspots is not constant. Over an average period of 11 years, sunspot numbers increase then decrease, approximately paralleling the increase and decrease in the Sun's overall activity. The fluctuation in sunspot numbers, a phenomenon recognized for over 150 years, is known as the sunspot cycle.

Solar activity peaked in 1989, when sunspots blotched the Sun's disk in greatest numbers. During that time, powerful flares were common, generating aurorae on Earth seen as far south as the Caribbean. One particular flare was so strong it produced a magnetic storm on March 10, 1989, that resulted in power blackouts throughout Quebec and Scandinavia.

Between 1996 and 1997, solar observations indicated a sporadic increase in solar activity and in sunspot numbers, but these reports were considered by astronomers to be more indicative of a prelude to renewed solar activity rather than its onset. Generally, the rise to sunspot maximum is more rapid than its decline. The new solar cycle now underway should translate into a decided increase in sunspot numbers during the first few years of the twenty-first century.

The onset of a new solar cycle can be discerned when the sunspot minimum of a previous cycle is nearing its end. A few new sunspots begin appearing around the lower-middle latitudes of the Sun, about 35° north and south. As the cycle continues, more and more sunspots migrate to lower latitudes until, at sunspot maxima, they form in bands in and around the equator.

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