SUNSPOTS are the sole sign of solar activity that is occasionally detectable to the eye. Known since antiquity, according to Greek sources, they were rediscovered by Europeans in 1611, after Galileo’s epochal application of the telescope to astronomical investigations. Despite the Greek knowledge, the European school of natural philosophy steadfastly held the Sun to be a perfect, unblemished sphere for more than a millennium. Please, do not look for sunspots yourself by positioning an instrument between your eyes and the Sun; it may cause you to go blind permanently-see the beginning of this chapter. It is not dangerous to look at the red disc of the Sun as it is about to set on a hazy horizon at sea level. On such occasions you may sometimes see large spots with the naked eye. (Do not use any instrument.)

Sunspots look like irregular holes in the Sun’s surface (figure 8.14). Although they appear to be dark areas, this is entirely a contrast effect. A large spot radiates as much light as the full Moon, but appears black against the brilliant photosphere. The black inner region is termed the UMBRA and the more luminous surrounding fringe the PENUMBRA. An average size for a spot would be 10 000km, but ones as big as 150 000km have been recorded.

It is usual for spots to occur in pairs or in-more complex groups. The larger spots are often associated with the smallest type of spot which is known as a PORE ; these range in size from about 3 000km down to the instrumental limit. Generally a pore only lasts a few hours, whereas true sunspots remain for a week or more. Large sunspots persist for several weeks, during which time solar rotation will take them out of view for a couple of weeks. Spectroscopy has shown that the temperature in the black region of a spot is 4 000 K, some 2000K less than the surrounding photosphere. Observations of sunspots approaching the limb of the Sun prove that they are depressions, not elevations, in the photosphere, since when a spot approaches the limb the near side becomes practically invisible ^whereas the far side is enlarged; this is the WILSON EFFECT.

Early explanations of sunspots seem bizarre today: some astronomers thought they might be planets inside the orbit of Mercury, and others that they were mountains poking through the photo¬sphere. Galileo opined that they were clouds, whereas Sir William Herschel speculated that they were holes in the fiery clouds through which one could see the dark, and presumably habitable, surface. Modern science provides a model that is simpler than any of these: sunspots are regions of unusually high magnetic field in which the photospheric temperature is 2000K cooler than average.

For nearly 250 years astronomers have kept worthwhile records of the number of spots visible on the Sun. They are the most readily observable tracer of the solar magnetic cycle. The number varies from day to day and year to year. About every 11 years the activity reaches a maximum; this is the period of the SUNSPOT CYCLE, first described in 1843. Conventionally, the sunspot cycle is recorded by means of an arbitrarily –defined quantity, the ZURICH SUNSPOT NUMBER Z
Z = C(S + 10G)

where S is the number of individual spots, G is the number of groups, and C is a correction factor designed to correct for varia¬tions in observer enthusiasm, equipment, and weather. The variation in sunspot numbers since 1700, and figure 8.16 displays recent cycles in more detail. Over the last 50 years the cycle time has averaged 10.4 years. It can be as short as 7 years or as long as 17 years. Besides the variations in the number of visible spots, another feature of the cycle is that at the start of the cycle, spots appear in the vicinity of latitudes +40° and —40°. As the cycle progresses, these two zones in which most spots are located migrate to within about 5° of the equator. At this stage the first spots of the next cycle erupt at high latitudes. A diagram which depicts the change in latitude during the cycle .

Spots are only one manifestation of the solar cycle. Many other features, such as the extent of the chromosphere and corona, or the frequency of solar flares, become more exaggerated or intense as the maximum of sunspot activity is approached. The sunspots are merely the most observable manifestation of solar activity, not the root cause of it.

Investigations of the magnetic fields of sunspots probably started in 1008. In that year George Hale noted that the spectral lines from the spots could be resolved into several components, each of which was polarized.The splitting of lines is caused in this ease by the ZEEMAN EFFECT, which occurs when atoms emit or absorb light in an intense magnetic field. Strong magnetic fields modify the energy levels in atoms and thus introduce extra structure into the spectral lines. The presence of Zeeman splitting provides a powerful means of probing the magnetism of sunspots because the separation of components of a given line depends on the field strength. This can be as high as 0.4 tesla (4000 gauss): this is thousands of times stronger than the geomagnetic field and may extend over an area exceeding the surface area of Earth. Such powerful fields cannot possibly be due to permanent magnets em¬bedded in the Sun, but must be caused by circulating electric cur-rents in the interior. Within the spot umbra, the field lines emerge more or less vertically.

Frequently sunspots appear as close pairs aligned parallel to the solar equator; these are called BIPOLAR SPOTS. Magnetic field measurements demonstrate that the two spots in a pair have opposite polarities: the field lines emerge from the surface at one spot and re-enter at the other, as shown in figure 8.19. During a particular sunspot cycle, and in a given solar hemisphere, the polarity of the western spot (the one leading in the direction of solar rotation) is always the same. In the other hemisphere the bipolar groups follow an analogous relation but the sense of the polarities is opposite (figure 8.20). This behavior persists through¬out the solar cycle; then, at the commencement of the next cycle, the polarities reverse in both hemispheres. We see that the magnetic behavior of the bipolar groups follows the full solar cycle of 22 years. In the case of large spots that are apparently magnetically isolated, because no spot of opposite polarity can be found nearby, it usually turns out that the magnetic field is much stronger in an adjacent area of the photosphere where, often, sunspots have formed and disappeared.

Sunspots are not vortices or tornadoes in the photosphere as some books suggest. The distinctive feature of a sunspot is the strong vertical magnetic field, and we shall use this property to make a model of sunspot behavior. The fact that differential rota¬tion builds up a shell of strong magnetic field under the photosphere has already been mentioned. Convection, in all probability, further twists and jumbles the field, as rising bubbles of conducting gas drag the field lines with them (figure 8.21). Kinks in the ropes of magnetic field lead to even stronger fields. Eventually the magnetic pressure is great enough to make the field buoyant: it wells up, bursts through the photosphere , and forms sunspots. Solar physicists have shown that this will first occur in latitudes ± 40°, because that is where the shearing forces are greatest; the eruptions lessen the field at higher latitudes and strengthen it nearer the equator. Therefore the spots gradually migrate to the equator, in accordance with the observations. The lower temperature inside sunspots arises partly because the intense magnetic field suppresses the ingress of new energy supplies from the convection zone and partly because the gas density is lowered as plasma flows out along the field lines.

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