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 Earth’s diameter—and may 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. At the end of the cycle, the number of spots drops to a minimum and the spots occur chiefly between about 5° and 15° north and south latitude.
How sunspots form.
Sunspots have magnetic fields of a strength up to 3,000 times as great as the average magnetic field of either the sun or 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 distort and twist 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 the surface because the kink both leaves and reenters the surface.
Another model for solar activity suggests that giant, doughnut-shaped rolls of turbulent gas rotate beneath the sun’s surface. These rolls encircle the sun and lie parallel to its equator. The gas in each carries a magnetic field and rotates perpendicularly to the ring of the rolls. Where gas in adjacent rolls pushes together, the magnetic field increases and sunspots arise.
The two members of a pair of sunspots have opposite magnetic polarities, much like the poles of a magnet. The two spots are called the preceding spot and the following spot because one “leads” the other in the direction of the sun’s rotation. During any given 11-year sunspot cycle, the magnetic polarity of sunspot pairs north of the solar equator is opposite to the polarity of the pairs south of the equator. For example, if the preceding spots in the Northern Hemisphere behave like the north-seeking end of a magnet, the preceding spots in the Southern Hemisphere behave like the south-seeking end. However, during the next sunspot cycle the behavior of the preceding spots is reversed. Thus, a complete sunspot cycle lasts about 22 years.
Other findings.
Astronomers have discovered that the sunspot cycle is only part of a more basic solar activity cycle, which includes solar flares, plages, and prominences. See Sun (Solar activity) . Such phenomena are closely associated with sunspots and occur in the region around the spots.
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 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 when a sunspot appears, the amount of energy reaching Earth decreases. But at sunspot maximum, when there are more sunspots, the sun is slightly brighter. This is because the region around the sunspot is brighter than the rest of the sun. Another satellite, the Solar and Heliospheric Observatory (SOHO), has revealed the structure of sunspots below the sun’s surface. SOHO is a joint European and U.S. satellite launched in 1995.
Scientists use data gathered by satellites and ground-based telescopes to predict the intensity of sunspot cycles. Sometimes, however, the cycles do not match these predictions. For example, the cycle that reached its maximum in 2014 was weaker than expected.
