THE SUNSPOT CYCLE
In the nineteenth century, German amateur astronomer Heinrich Schwabe was systematically scanning the Sun's surface, hoping to catch in transit a planet then thought to be orbiting inside Mercury.
To avoid mistakes, he tabulated all the sunspots he saw. Schwabe never found the planet—which does not, in fact, exist—but he did make the discovery that the number of sunspots rose and fell in a pattern lasting about a decade.
Today, after observation over many years, scientists have determined that sunspots follow an 11-year cycle, during which the number of spots goes from a minimum to a maximum. Solar astronomer reckon Cycle 0 as being the cycle that peaked in 1750. On this reckoning, Cycle 22 peaked in mid-1989 and end in late 1996, and Cycle 23 is expected to peak around 2001. Each cycle has an asymmetric shape, rising to maximum activity in about five years and declining over about six years.
At the start of a new cycle, the first spots appear around latitudes 35 degrees north and south. As the cycle progresses, sunspots become more numerous and appear at latitudes nearer the Sun's equator. By the end of the cycle, sunspots appear around 7 degrees north and south latitude. As one cycle is tailing oft, the first spots of the next cycle are already appearing at high latitudes.
Sunspots are highly magnetized, and astronomers have discovered that there is actually a 22-year magnetic cycle superimposed on the ordinary 11-year cycle. When they measure the magnetic fields of sunspots during any 11-year cycle, they find that the spots leading each group across the disk show opposite magnetic polarities in the two hemispheres. Then, in the next 11-year cycle, all leading spots switch polarity.