Chapter 15--Our Sun - Geological Sciences

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0.5% percentage of Sun’s surface covered by sunspots 0.4% 0.3% 0.2% 0.1% 0.0% 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 a This graph shows how the number of sunspots on the Sun changes with time. The vertical axis shows the percentage of the Sun’s surface covered by sunspots. The cycle has a period of approximately 11 years. year 90 N 30 N latitude Equator 30 S 90 S 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 b This graph shows how the latitudes at which sunspot groups appear tend to shift during a single sunspot cycle. Each dot represents a group of sunspots and indicates the year (horizontal axis) and latitude (vertical axis) at which the group appeared. Figure 15.21 Sunspot cycle during the past century. year What Causes the Sunspot Cycle? The causes of the Sun’s magnetic fields and the sunspot cycle are not well understood, but we believe we know the general nature of the processes involved. Convection is thought to dredge up weak magnetic fields generated in the solar interior, amplifying them as they rise. The Sun’s rotation—faster at its equator than near its poles—then stretches and shapes these fields. Imagine what happens to a magnetic field line that originally runs along the Sun’s surface directly from the north pole to the south pole. At the equator, the field line circles the Sun in 25 days, but at higher latitudes the field line lags behind. Gradually, this rotation pattern winds the field line more and more tightly around the Sun (Figure 15.22). This process, operating at all times over the entire Sun, produces the contorted field lines that generate sunspots and other solar activity. Investigating how the Sun’s magnetic field develops and changes in time requires sophisticated computer models. Scientists are working hard on such models, but the behavior of these fields is so complex that approximations are necessary even with the best supercomputers. Using these computer models, scientists have successfully replicated some features of the sunspot cycle, such as changes in the number and latitude of sunspots and the magnetic field reversals that occur about every 11 years. However, much still remains mysterious, including why the period of the sunspot cycle varies and why solar activity is different from one cycle to the next. Solar Activity and Earth During solar maximum, solar flares and other forms of solar activity send large numbers of highly energetic charged particles (protons and electrons) toward Earth. Sometimes these particles travel in the smooth flow known as the solar wind. Other times they come in the form of huge magnetic bubbles called coronal mass ejections. Do these forms of solar weather affect Earth? In at least some ways, the answer is a definitive yes. 514 part V • Stellar Alchemy
e quator N N N magnetic field line differing rotation rates time time S S S Figure 15.22 The Sun rotates more quickly at its equator than it does near its poles. Because gas circles the Sun faster at the equator, it drags the Sun’s north-south magnetic field lines into a more twisted configuration. The magnetic field lines linking pairs of sunspots, depicted here as green and black blobs, trace out the directions of these stretched and distorted field lines. The magnetic field associated with the solar wind constantly interacts with Earth’s magnetic field. Occasionally these fields interconnect. When that happens, large amounts of energy are released from the magnetic field into the charged particles near the interconnection zone. Many of these energized particles then flow down Earth’s magnetic field lines toward the poles (Figure 15.23a). Collisions between the charged particles and atoms in Earth’s upper atmosphere cause electrons in the atoms to jump to higher energy levels [Section 4.4].These excited atoms subsequently emit visible-light photons as they drop to lower energy levels, creating the shimmering light of auroras (Figure 15.23b). Because coronal mass ejections are particularly energetic, the auroras they stimulate can be especially spectacular. Particles streaming from the Sun after the occurrence of solar flares, coronal mass ejections, or other major solar storms can also have practical impacts on society. For example, these particles can hamper radio communications, disrupt electrical power delivery, and damage the electronic components in orbiting satellites. During a particularly powerful magnetic storm on the Sun in March 1989, the U.S. Air Force temporarily lost track of over 2,000 satellites, SPECIAL TOPIC Long-Term Change in Solar Activity Figure 15.21 shows that the sunspot cycle varies in length and intensity, and it sometimes seems to disappear altogether. With these facts as background, many scientists are searching for longerterm patterns in solar activity. Unfortunately, the search for longerterm variations is difficult because telescopic observations of sunspots cover a period of only about 400 years. Some naked-eye observations of sunspots recorded by Chinese astronomers go back almost 2,000 years, but these records are sparse, and nakedeye observations may not be very reliable. We can also guess at past solar activity from descriptions of solar eclipses recorded around the world: When the Sun is more active, the corona tends to have longer and brighter “streamers” visible to the naked eye. Another way to gauge past solar activity is to study the amount of carbon-14 in tree rings. High-energy cosmic rays [Section 19.2] coming from beyond our own solar system produce radioactive carbon-14 in Earth’s atmosphere. During periods of high solar activity, the solar wind tends to grow stronger, shielding Earth from some of these cosmic rays. Thus, production of carbon-14 drops when the Sun is more active. All the while, trees steadily breathe in atmospheric carbon, in the form of carbon dioxide, and incorporate it year by year into each ring. We can therefore estimate the level of solar activity in any given year by measuring the level of carbon-14 in the corresponding ring. No clear evidence has yet been found of longer-term cycles of solar activity, but the search goes on. Theoretical models predict a very long term trend of lessening solar activity. According to our theory of solar system formation, the Sun must have rotated much faster when it was young [Section 9.3].Because a combination of convection and rotation generates solar activity, a faster rotation rate should have meant much more activity. Observations of other stars that are similar to the Sun but rotate faster confirm that these stars are much more active. We find evidence for many more “starspots” on these stars than on the Sun, and their relatively bright ultraviolet and X-ray emissions suggest that they have brighter chromospheres and coronas—just as we would expect if they are more active than the Sun. chapter 15 • Our Star 515
Page 1 and 2: PART V STELLAR ALCHEMY
Page 3 and 4: I say Live, Live, because of the Su
Page 5 and 6: Table 15.1 Basic Properties of the
Page 7 and 8: COMMON MISCONCEPTIONS The Sun Is No
Page 9 and 10: in the creation of two gamma-ray ph
Page 11 and 12: Figure 15.9 Vibrations on the surfa
Page 13 and 14: Figure 15.11 This tank of dry-clean
Page 15 and 16: a This schematic diagram shows how
Page 17 and 18: weaker magnetic field stronger magn
Page 19: Figure 15.20 An X-ray image of the
Page 23 and 24: THE BIG PICTURE Putting Chapter 15
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Chapter 15--Our Sun - Geological Sciences

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