A Sunspot is a relatively dark area on the surface of the sun. Sunspots appear dark because they are cooler than the rest of the sun’s visible surface. They may have a temperature of only about 7000 °F (4000 °C), compared with 11,000 °F (6000 °C) for their surroundings.
A typical large sunspot may have a diameter of about 20,000 miles (32,000 kilometers)–several times larger than the earth’s diameter–and last for months. Such a large spot consists of a dark central region called the umbra and a lighter surrounding region known as the penumbra. A very small sunspot, known as a pore, has no penumbra. Pores may be several hundred miles in diameter, and they may last only for hours.
The number of sunspots and solar latitudes at which they appear vary over a period of about 11 years. This period is called the sunspot cycle. At the beginning of a cycle, sunspots appear chiefly between 20° and 40° north and south of the sun’s equator. Later, the spots increase in number and occur closer to the solar equator. By the time the sunspots are greatest in number, they lie primarily between 5° and 40° north and south latitude.
Sunspots have magnetic fields of a strength up to 3,000 times as great as the average magnetic field of either the sun or the earth. Astronomers believe the cause of sunspots is closely related to this fact. According to a standard explanation, the strong magnetic fields of the sun have the shape of tubes just below the solar surface at the beginning of a sunspot cycle. These tubes lie perpendicular to the sun’s equator. The sun rotates faster at its equator than at its poles, and so the tubes are stretched out in the east-west direction. Kinks then develop in the magnetic tubes and push through the solar surface. A pair of sunspots appears wherever a kink penetrates, because the kink both leaves and reenters the surface.
solar activity cycle.
In the late 1890′s, E. Walter Maunder, a British astronomer, concluded that no sunspots occurred from 1645 to 1715. Research during the 1970′s showed that only a small number of sunspots occurred in those 70 years. The existence of that period, called the Maunder minimum, indicates that the sunspot cycle may not be as basic a property of the sun as astronomers had thought. Some research has shown that certain aspects of the earth’s weather might be linked to solar activity. But these studies remain inconclusive.
From 1980 to 1989, a United States satellite called Solar Maximum Mission studied solar activity. Its data showed that changes in the amount of solar energy reaching the earth’s atmosphere correspond to changes in the amount of the sun’s surface covered by sunspots. For example, when a large sunspot appears, the amount of energy reaching the atmosphere decreases. But on a longer time scale, the sun is overall slightly brighter at the time of a sunspot maximum–which corresponds to the maximum of the solar activity cycle.