Sunspots and Sunspot Groups
Dark features on the Sun have been reported for at least 2000 years. Several sightings per century are contained in ancient Chinese records. Although not the first sightings recorded in Europe, Galileo's telescopic observations in 1610 provided the first details on sunspots, the most conspicuous of a number of transient phenomena to be found in the solar atmosphere.
A typical sunspot has a cellular structure with a dark center, the umbra, surrounded by a grayish filamentary region, the penumbra. Although sunspots emit radiation, the umbra looks dark because it is seen against an even brighter photospheric background, whose temperature is some 1800 K higher. The umbra is about one-fourth as bright as the photosphere and the penumbra about three-fourths as bright.
Sunspots develop in a matter of hours as small pores in the intergranular region of the photosphere. They grow rapidly, and they generally form in clusters, marking a sunspot group, whose orientation is approximately parallel to the solar equator. Each end of the group is often dominated by a large spot surrounded by smaller spots. The very largest groups may cover up to one-fifth the solar diameter. Sometimes a sunspot group persists for several months, but a typical lifetime is about 1 week. A typical large spot in a group is some 10,000 km across; exceptional ones are 50,000 km in diameter, or about four times the diameter of the Earth. In a week or so this large spot builds to its maximum diameter; then its size slowly declines. Individual spots in a sunspot group undergo slow changes from day to day while they maintain their association.
Sunspot Cycle
More than a century ago it was discovered that sunspots come and go in a roughly 11 year cycle. A graph of sunspot number verses time shows many cycles of this ll year variation. Such a figure shows that the heights of successive maxima are obviously unequal, and the interval between successive peaks or troughs is not constant; the 11-year period is a very rough average. Each sunspot cycle opens with spots forming at latitudes around 35oN and 35oS, and as the cycle progresses, spots form closer to the equator in both hemispheres. The maximum number of spots form when sunspots are forming at latitudes around 25o in both hemispheres. When the last spots of a cycle are forming near 5oN or 5oS, a few spots again form at latitudes around 35oN or 35oS herald the beginning of a new cycle.
Magnetic Fields in Sunspots
Immense arching, curved features are observe around sunspot groups (Figure 15.8d). They are structures of gas whose shape is determined by curved magnetic field lines, since sunspots are known to be the centers of intense magnetic fields. Astronomers know this because of the Zeeman effect (Section 12.5). It was first identified in the absorption lines of sunspot spectra by George Ellery Hale (1868-1938) at the Mount Wilson Observatory in 1908. In the Zeeman effect, the strength of the magnetic field can be determined from the separation of components of absorption lines, while the direction of the field is shown by the sense of polarization of these components (Figure 15.5).
The leading spots of a spot group (in the forward direction of the Sun's rotation) are opposite in polarity from the following spots. Opposite polarities are like those of the north and south ends of a bar magnet. The unit of measure of the intensity or strength of the magnetic field is called a gauss. The measured field strength in sunspots exceeds that of the Earth's field by several thousand times, being several thousand gauss. In the Sun's northern and southern hemispheres, polarities of leading and following spots are also opposite to each other. That is, the polarity of the leading spot in the northern hemisphere may be northseeking in one sunspot cycle, whereas that for the southern hemisphere is southseeking. Then in the next sunspot cycle the leading spot will be southseeking in the northern and northseeking in the southern hemisphere. Thus magnetic field polarity reverses in both hemispheres in succeeding sunspot cycles.
As a whole, the Sun does not have a general magnetic field like that of the Earth. But by averaging over small localized and intense magnetic fields, one gets the impression of a general field a few times stronger than the Earth's field, or several gauss. Magnetic measurements have been made for the Sun fairly regularly over the last 25 years. From which it has been found that magnetic fields in the polar regions reverse polarity near the time of sunspot maxima. Thus what appears to be a general field is probably the accumulation of surface fields that have drifted into the polar regions.
Origin of Sunspots
Although astronomers have explanations for sunspot behavior and the magnetic field phenomena that go with it, these explanations are not completely satisfying in all respects. A paramount point in these explanations is the answer to the question of why sunspots are cooler than their surroundings and how they are able to maintain that condition for so long a period of time. Since energy flows from hotter to cooler regions, a flood of photons from high-temperature gas surrounding sunspots should flow into the cooler spot and eliminate very quickly any difference in temperature. The reason that this does not happen appears to be the ability of magnetic fields that exist in the interiors of spots to inhibit the flow of thermal energy into the spot.
The most acceptable theory of sunspots asserts that the interaction of the Sun's differential rotation and convective motions in the hydrogen convection zone below the photosphere generates and then distort a sub-photospheric magnetic field. As the magnetic field lines are pulled and stretched unequally at different latitudes by the flow of gas to which they are attached (maximum stretching is at the equator, where rotation is swiftest), the field lines eventually curl into tubes below the photosphere. If a kink develops, a tube of magnetic field lines is buoyed upward, so that it arches into the photosphere, forming a bipolar sunspot group.
As a bipolar sunspot group breaks up and decays, the field of the following spots migrates toward polar regions and eventually neutralizes the existing polar field, allowing a new one to form, its polarity reversed from the previous cycle. The period of complete reversal of sunspot polarity is thus 22 years. This magnetic cycle is more regular than the 11-year cycle. That is, if one sunspot cycle were to be 12 years in length, then the following cycle would be close to 10 years so that their sum would be the 22 years of the magnetic cycle.