The Royal Observatory, Greenwich in London has kept detailed records of the size and location of sunspots since 1874. That data shows that sunspots don't appear randomly on the sun's surface, but instead are concentrated in two latitude bands on each side of the equator. The frequency, location and intensity of sunspots within those bands also changes over time. Roughly every 11 years, the number of sunspots increases from nearly zero to more than 100, and then decreases to near zero again as a new cycle starts. This pattern is called the sunspot cycle. Since 1700, sunspot cycles have varied in length between nine and 14 years. At the beginning of the cycle, sunspots form in the sun's mid-latitudes. But as the cycle progresses, they occur closer to the equator.
According to a model recently developed by astronomer Mausumi Dikpati and his colleagues, the cycle occurs because the Sun contains a sort of conveyor belt that circulates a hot, electrified gas called plasma between the sun's equator and its poles and then back again over a period of years. When sunspots formed early in a sunspot cycle begin to decay, they leave a kind of magnetic imprint on the moving plasma beneath them. The conveyor belt then carries the plasma with those magnetic imprints toward the poles and then back inside the sun. The magnetic fields within the sun are distorted and intensified even more before the conveyor belt sends the plasma to resurface near the equator. There, the plasma forms new, even more powerful sunspots.
Sunspot cycles are asymmetrical, with sunspot activity rising quickly at the beginning of a cycle and declining gradually after that. The point where sunspots appear and reach their peak of intensity is called the solar maximum, ("solar max" for short). The low point, where there's hardly any sunspot activity, is called the solar minimum. Solar minimums usually last for several years, but they sometimes endure for much longer. For example, there was a period between 1650 and 1710 -- called the Maunder minimum, after the astronomer who discovered it -- when there was little or no sunspot activity for 60 years. (The cause remains a mystery, though some astronomers have theorized that other stars occasionally go through such long dormant periods as well.) Maunder also plotted sunspot activity over time on a graph, producing a butterfly-shaped diagram in which sunspot cycles looked like the butterfly's wings. Scientists continue to work on ways to predict sunspot activity, so we can better cope with its potentially disruptive effects on power grids and radio communications.
In the next section, we'll look at the effects that sunspots have on Earth.