As he worked away patiently with his examination of sunspots, Maunder found a pattern emerging. In 1843 the German astronomer Samuel Heinrich Schwabe (1789-1875) had discovered that the number of sunspots increases and decreases over a regular 11-year cycle. Maunder found that the solar latitudes in which sunspots emerge change in a regular way over the course of the sunspot cycle. The first ones appear some distance from the solar equator and later ones move gradually closer to the equator (see the sidebar).
Maunder did not spend all of his time photographing the Sun and studying the sunspots. He also searched the old records in the observatory library to discover what had already been learned about the sunspot cycle.
The 11-year sunspot cycle
Years ago, the English astronomer William Herschel (1738-1822) had suspected there might be a link between the number of sunspots and the weather. In 1801 he attempted to link the number of sunspots each year to the price of grain. Grain prices rise when the weather is bad and the harvest poor and fall when the harvest is good.
The visible surface of the Sun, called the photosphere, is a layer of gas several hundreds of miles thick and with a temperature of about 9,900°F (5,500'C). From time to time, irregular dark patches are visible on the photosphere. These are sunspots. They are areas up to 31,070 miles (50,000 km) across where the temperature is about 2,700°F (1,500'C) cooler than their surroundings. Most sunspots have a dark center,
called the umbra, surrounded by a paler penumbra. Each sunspot is surrounded by a facula. Faculae are very bright regions of the photosphere, where the temperature is higher than it is elsewhere—facula means "torch." The illustration shows the appearance of sunspots, although in order to do so clearly it greatly exaggerates their size.
Sunspots usually appear in clusters, each sunspot lasting for about two weeks, and the number of sunspots increases and decreases over a regular 11-year sunspot cycle. The cycle begins with a period of several weeks during which no sunspots are visible. Then they start to appear at latitudes 30°-40° in both hemispheres. Over the succeeding five years the sunspots appear and disappear continually, but their number increases steadily and they move closer to the solar equator. At the end of five years the number of sunspots reaches a maximum. After that the number declines over the next six years, but the sunspots continue to move closer to the equator. When they reach about latitude 7° the sunspots fade gradually. Then the cycle resumes, 11 years after the commencement of the preceding cycle.
Sunspots are caused by intense magnetic fields below the solar surface. These suppress the convection currents that carry hot gases to the top of the photosphere. The intensity of the stream of charged particles emitted by the Sun, known as the solar wind, is linked to the sunspot cycle. The solar wind intensifies as the number of sunspots increases. The intensity of the solar wind affects the intensity of both the cosmic radiation and ultraviolet radiation reaching the Earth.
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