Chapter 15--Our Sun - Geological Sciences

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Figure 15.18 A gigantic solar prominence erupts from the solar surface at the upper right of this ultraviolet-light photo (from the SOHO mission). The gas within this prominence, which is over 20 times the size of Earth, is quite hot but still cooler than the million-degree gas of the surrounding corona. UV Figure 15.19 This photo (from TRACE) of ultraviolet light emitted by hydrogen atoms shows a solar flare erupting from the Sun’s surface. UV from the Sun. Figure 15.20 shows an X-ray image of the Sun. As the solar heating model predicts, the brightest regions of the corona tend to be directly above sunspot groups. Some regions of the corona, called coronal holes, barely show up in X-ray images. More detailed analyses show that the magnetic field lines in coronal holes project out into space like broken rubber bands, allowing particles spiraling along them to escape the Sun altogether. These particles streaming outward from the corona constitute the solar wind, which blows through the solar system at an average speed of about 500 kilometers per second and has important effects on planetary surfaces, atmospheres, and magnetospheres. Well beyond the planets, the pressure of interstellar gas must eventually halt the solar wind. The Pioneer and Voyager spacecraft that visited the outer planets in the 1970s and 1980s are still traveling outward from our solar system and may soon encounter this “boundary” (called the heliopause) of the realm of the Sun. The solar wind also gives us something tangible to study. In the same way that meteorites provide us with samples of asteroids we’ve never visited, solar wind particles captured by satellites provide us with a sample of material from the Sun. Analysis of these solar particles has reassuringly verified that the Sun is made mostly of hydrogen, just as we conclude from studying the Sun’s spectrum. 512 part V • Stellar Alchemy
Figure 15.20 An X-ray image of the Sun reveals the million-degree gas of the corona. Brighter regions of this image (yellow) correspond to regions of stronger X-ray emission. The darker regions are the coronal holes from which the solar wind escapes. (From the Yohkoh space observatory.) X-ray 15.5 Solar Weather and Climate Individual sunspots, prominences, and flares are short-lived phenomena, somewhat like storms on Earth. They constitute what we call solar weather or solar activity.You know from personal experience that the Earth’s weather is notoriously unpredictable. The same is true for the Sun: We cannot predict precisely when or where a particular sunspot or flare will appear. Earth’s climate, on the other hand, is quite regular from season to season. So it is with the Sun, where despite day-to-day variations the general nature and intensity of solar activity follow a predictable cycle. The Sunspot Cycle Long before we realized that sunspots were magnetic disturbances, astronomers had recognized patterns in sunspot activity. The most notable pattern is the number of sunspots visible on the Sun at any particular time. Thanks to telescopic observations of the Sun recorded by astronomers since the 1600s, we know that the number of sunspots gradually rises and falls in a sunspot cycle with an average period of about 11 years (Figure 15.21a). At the time of solar maximum,when sunspots are most numerous, we may see dozens of sunspots on the Sun at one time. In contrast, we see few if any sunspots at the time of solar minimum.The frequency of prominences and flares also follows the sunspot cycle, with these events being most common at solar maximum and least common at solar minimum. Although we’ll call it an “11-year” cycle, the interval between solar maxima is sometimes as long as 15 years or as short as 7 years. The number of sunspots also varies dramatically (Figure 15.21a). In fact, sunspot activity virtually ceased between the years 1645 and 1715, a period sometimes called the Maunder minimum (after E. W. Maunder, who identified it in historical sunspot records). Another feature of the sunspot cycle is a gradual change in the solar latitudes at which individual sunspots form and dissolve (Figure 15.21b). As a cycle begins at solar minimum, sunspots form primarily at mid-latitudes (30° to 40°) on the Sun. The sunspots tend to form at lower latitudes as the cycle progresses, appearing very close to the solar equator as the next solar minimum approaches. A less obvious feature of the sunspot cycle is that something peculiar happens to the Sun’s magnetic field at each solar minimum. The field lines connecting all pairs of sunspots (see Figure 15.17) tend to point in the same direction throughout an 11-year solar cycle (within each hemisphere). For example, all compass needles might point from the easternmost sunspot to the westernmost sunspot in a pair. However, as the cycle ends at solar minimum, the magnetic field reverses: In the subsequent solar cycle, the field lines connecting pairs of sunspots point in the opposite direction. Apparently, the entire magnetic field of the Sun flip-flops every 11 years. The magnetic reversals hint that the sunspot cycle is related to the generation of magnetic fields on the Sun. They also tell us that the complete magnetic cycle of the Sun, called the solar cycle, really averages 22 years, since it takes two 11-year cycles before the magnetic field is back the way it started. chapter 15 • Our Star 513
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: weaker magnetic field stronger magn
Page 21 and 22: e quator N N N magnetic field line
Page 23 and 24: THE BIG PICTURE Putting Chapter 15
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Chapter 15--Our Sun - Geological Sciences

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