16-9 Sunspots are produced by a 22-year cycle in the Sun’s magnetic field

Why should the number of sunspots vary with an 11-year cycle? Why should their average latitude vary over the course of a cycle? And why should sunspots exist at all? The first step toward answering these questions came in 1908, when the American astronomer George Ellery Hale discovered that sunspots are associated with intense magnetic fields on the Sun.

Probing Solar Magnetism

When Hale focused a spectroscope on sunlight coming from a sunspot, he found that many spectral lines appear to be split into several closely spaced lines (Figure 16-21). This “splitting” of spectral lines is called the Zeeman effect, after the Dutch physicist Pieter Zeeman, who first observed it in his laboratory in 1896. Zeeman showed that a spectral line splits when the atoms are subjected to an intense magnetic field. The more intense the magnetic field, the wider the separation of the split lines.

Figure 16-21: R I V U X G
Sunspots Have Strong Magnetic Fields (a) A black line in this image of a sunspot shows where the slit of a spectrograph was aimed. (b) This is a portion of the resulting spectrum, including a dark absorption line caused by iron atoms in the photosphere. The splitting of this line by the sunspot’s magnetic field can be used to calculate the field strength. Typical sunspot magnetic fields are over 5000 times stronger than Earth’s field at its north and south poles.

(NOAO)

Hale’s discovery showed that sunspots are places where the hot gases of the photosphere are bathed in a concentrated magnetic field. Many of the atoms of the Sun’s atmosphere are ionized due to the high temperature. The solar atmosphere is thus a special type of gas called a plasma, in which electrically charged ions and electrons can move freely. Like any moving, electrically charged objects, they can be deflected by magnetic fields. Figure 16-22 shows how a magnetic field in the laboratory bends a beam of fast-moving electrons into a curved trajectory. Similarly, the paths of moving ions and electrons in the photosphere are deflected by the Sun’s magnetic field. In particular, magnetic forces act on the hot plasma that rises from the Sun’s interior due to convection. Where the magnetic field is particularly strong, these forces push the hot plasma away. The result is a localized region where the gas is relatively cool and thus glows less brightly—in other words, a sunspot.

Figure 16-22: R I V U X G
Magnetic Fields Deflect Moving, Electrically Charged Objects In this laboratory experiment, a beam of negatively charged electrons (shown by a blue arc) is aimed straight upward from the center of the apparatus. The entire apparatus is inside a large magnet, and the magnetic field deflects the beam into a curved path.

(Andrew Lambert Photography/Science Source)

To get a fuller picture of the Sun’s magnetic fields, astronomers take images of the Sun at two wavelengths, one just less than and one just greater than the wavelength of a magnetically split spectral line. From the difference between these two images, they can construct a picture called a magnetogram, which displays the magnetic fields in the solar atmosphere. Figure 16-23a is an ordinary white-light photograph of the Sun taken at the same time as the magnetogram in Figure 16-23b. In the magnetogram, dark blue indicates areas of the photosphere with one magnetic polarity (north), and yellow indicates areas with the opposite (south) magnetic polarity. This image shows that many sunspot groups have roughly comparable areas covered by north and south magnetic polarities (see also Figure 16-23c). Thus, a sunspot group resembles a giant bar magnet, with a north magnetic pole at one end and a south magnetic pole at the other.

Figure 16-23: R I V U X G
Mapping the Sun’s Magnetic Field (a) This visible-light image and (b) this false-color magnetogram were recorded at the same time. Dark blue and yellow areas in the magnetogram have north and south magnetic polarity, respectively; blue-green regions have weak magnetic fields. The highly magnetized regions in (b) correlate with the sunspots in (a). (c) The two ends of this large sunspot group have opposite magnetic polarities (colored blue and yellow), like the ends of a giant bar magnet.

(NOAO)

If different sunspot groups were unrelated to one another, their magnetic poles would be randomly oriented, like a bunch of compass needles all pointing in random directions. As Hale discovered, however, there is a striking regularity in the magnetization of sunspot groups. As a given sunspot group moves with the Sun’s rotation, the sunspots in front are called the “preceding members” of the group. The spots that follow behind are referred to as the “following members.” Hale compared the sunspot groups in the two solar hemispheres, north or south of the Sun’s equator. He found that the preceding members in one solar hemisphere all have the same magnetic polarity, while the preceding members in the other hemisphere have the opposite polarity. Furthermore, in the hemisphere where the Sun has its north magnetic pole, the preceding members of all sunspot groups have north magnetic polarity. In the opposite hemisphere, where the Sun has its south magnetic pole, the preceding members all have south magnetic polarity.

Along with his colleague Seth B. Nicholson, Hale also discovered that the Sun’s polarity pattern completely reverses itself every 11 years—the same interval as the time from one solar maximum to the next. The hemisphere that has preceding north magnetic poles during one 11-year sunspot cycle will have preceding south magnetic poles during the next 11-year cycle, and vice versa. The north and south magnetic poles of the Sun itself also reverse every 11 years. Thus, the Sun’s magnetic pattern repeats itself only after two sunspot cycles, which is why astronomers speak of a 22-year solar cycle.

The Magnetic-Dynamo Model

In 1960, the American astronomer Horace Babcock proposed a description that seems to account for many features of this 22-year solar cycle. Babcock’s scenario, called a magnetic-dynamo model, makes use of two basic properties of the Sun’s photosphere—differential rotation and convection. Differential rotation causes the magnetic field in the photosphere to become wrapped around the Sun (Figure 16-24). As a result, the magnetic field becomes concentrated at certain latitudes on either side of the solar equator. Convection in the photosphere creates tangles in the concentrated magnetic field, and “kinks” erupt through the solar surface. Sunspots appear where the magnetic field protrudes through the photosphere. The theory suggests that sunspots should appear first at northern and southern latitudes and later form nearer to the equator, which is just what is observed (see Figure 16-20). Note also that as shown on the far right in Figure 16-24, the preceding member of a sunspot group has the same polarity (N or S) as the Sun’s magnetic pole in that hemisphere, which is just as Hale observed.

Figure 16-24: Babcock’s Magnetic Dynamo Model Magnetic field lines tend to move along with the plasma in the Sun’s outer layers. Because the Sun rotates faster at the equator than near the poles, a field line that starts off running from the Sun’s north magnetic pole (N) to its south magnetic pole (S) ends up wrapped around the Sun like twine wrapped around a ball. The insets on the far right show how sunspot groups appear where the concentrated magnetic field rises through the photosphere.

Differential rotation eventually undoes the twisted magnetic field. The preceding members of sunspot groups move toward the Sun’s equator, while the following members migrate toward the poles. Because the preceding members from the two hemispheres have opposite magnetic polarities, their magnetic fields cancel each other out when they meet at the equator. The following members in each hemisphere have the opposite polarity to the Sun’s pole in that hemisphere; hence, when they converge on the pole, the following members first cancel out and then reverse the Sun’s overall magnetic field. The fields are now completely relaxed. Once again, differential rotation begins to twist the Sun’s magnetic field, but now with all magnetic polarities reversed. In this way, Babcock’s model helps to explain the change in field direction every 11 years.

Figure 16-25: Rotation of the Solar Interior This cutaway picture of the Sun shows how the solar rotation period (shown by different colors) varies with depth and latitude. The surface and the convective zone have differential rotation (a short period at the equator and longer periods near the poles). Deeper within the Sun, the radiative zone seems to rotate like a rigid sphere.

(Courtesy of K. Libbrecht, Big Bear Solar Observatory)

Recent discoveries in helioseismology (Section 16-3) offer new insights into the Sun’s magnetic field. By comparing the speeds of sound waves that travel with and against the Sun’s rotation, helioseismologists have been able to determine the Sun’s rotation rate at different depths and latitudes. As shown in Figure 16-25, the Sun’s surface pattern of differential rotation persists through the convective zone. Farther in, within the radiative zone, the Sun seems to rotate like a rigid object with a period of 27 days at all latitudes. Astronomers suspect that the Sun’s magnetic field originates in a relatively thin layer where the radiative and convective zones meet and slide past each other due to their different rotation rates.

One dilemma about sunspots is that compressed magnetic fields tend to push themselves apart, which means that sunspots should dissipate rather quickly. Yet observations show that sunspots can persist for many weeks. The resolution of this paradox may have been found using helioseismology (see Section 16-3). Analysis of the vibrations of the Sun around sunspots shows that beneath the surface of the photosphere, the gases surrounding each sunspot are circulating at high speed—rather like a hurricane as large as Earth. The circulation of charged gases around the magnetic field holds the fields in place, thus stabilizing the sunspot.

Helioseismology—the analysis of solar sound waves—can also take advantage of the relationship between sunspots and the solar magnetic field. Compared to typical solar material, sunspots absorb solar sound waves more strongly. When sunspots occur on the opposite side of the Sun, their increased absorption means that interior sound waves do not reflect back as strongly through the body of the Sun. Amazingly, this effect is used to estimate sunspot activity on the opposite side of the Sun. Knowing the number of sunspots facing away from Earth is actually useful: Added to the sunspots facing Earth, the total sunspot activity is continuously monitored to help forecast space weather—these are variations in the solar wind and magnetic field that can affect satellites and astronauts.

Much about sunspots and solar activity remains mysterious. There are perplexing irregularities in the solar cycle. For example, the overall reversal of the Sun’s magnetic field is often piecemeal and haphazard. One pole may reverse polarity long before the other. For several weeks the Sun’s surface may have two north magnetic poles and no south magnetic pole at all.

Furthermore, there seem to be times when all traces of sunspots and the sunspot cycle vanish for many years. For example, virtually no sunspots were seen from 1645 through 1715. Curiously, during these same years Europe experienced record low temperatures, often referred to as the Little Ice Age, whereas the western United States was subjected to severe drought. By contrast, there was apparently a period of increased sunspot activity during the eleventh and twelfth centuries, during which Earth was warmer than it is today. Thus, variations in solar activity appear to affect climates on Earth. The origin of this Sun-Earth connection is a topic of ongoing research.