Chapter 15--Our Sun - Geological Sciences

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a This close-up view of the Sun’s surface (right) shows two large sunspots and several smaller ones. Both of the big sunspots are roughly as large as Earth. Figure 15.15 Sunspots are regions of intense magnetic activity. VIS tight magnetic field lines suppress convection within each sunspot and prevent surrounding plasma from sliding sideways into the sunspot. With hot plasma unable to enter the region, the sunspot plasma becomes cooler than that of the rest of the photosphere (Figure 15.17a). The magnetic field lines connecting two sunspots often soar high above the photosphere, through the chromosphere, and into the corona (Figure 15.17b). These vaulted loops of magnetic field sometimes appear as solar prominences, in which the field traps gas that may glow for days or even weeks. Some prominences rise to heights of more than 100,000 kilometers above the Sun’s surface (Figure 15.18). The most dramatic events on the solar surface are solar flares,which emit bursts of X rays and fast-moving charged particles into space (Figure 15.19). Flares generally occur in the vicinity of sunspots, leading us to suspect that they occur when the magnetic field lines become so twisted and knotted that they can no longer bear the tension. The magnetic field lines suddenly snap like tangled elastic bands twisted beyond their limits, releasing a huge amount of energy. This energy heats the nearby plasma to 100 million K over the next few minutes to few hours, generating X rays and accelerating some of the charged particles to nearly the speed of light. The Chromosphere and Corona The high temperatures of the chromosphere and corona perplexed scientists for decades. After all, temperatures gradually decline as we move outward from the core to the VIS b Spectra of sunspots can be used to measure the strength of their magnetic fields. This image shows the spectrum of a sunspot and its surroundings. The sunspot region shows up as dark horizontal bands because it is darker than the rest of the solar surface in its vicinity. The vertical bands are absorption lines that are present both inside and outside the sunspots. The influence of strong magnetic fields within the sunspot region splits a single absorption line into three parts. Measuring the separation between these lines tells us the strength of the magnetic field within the sunspot. top of the photosphere. Why should this decline suddenly reverse? Some aspects of this atmospheric heating remain a mystery today, but we have at least a general explanation: The Sun’s strong magnetic fields carry energy upward from the churning solar surface to the chromosphere and corona. More specifically, the rising and falling of gas in the convection zone probably shakes magnetic field lines beneath the solar surface. This shaking generates waves along the magnetic field lines that carry energy upward to the solar 510 part V • Stellar Alchemy
weaker magnetic field stronger magnetic field e e e weaker magnetic field a Magnetic field lines follow b Lines closer together c Charged particles follow the directions that compass indicate a stronger field. paths that spiral along needles would point. magnetic field lines. e Figure 15.16 We draw magnetic field lines to represent invisible magnetic fields. atmosphere. Precisely how the waves deposit their energy in the chromosphere and corona is not known, but the waves agitate the low-density plasma of these layers, somehow heating them to high temperatures. Much of this heating appears to happen near where the magnetic field lines emerge from the Sun’s surface. According to this model of solar heating, the same magnetic fields that keep sunspots cool make the overlying plasma of the chromosphere and corona hot. We can test this idea observationally. The gas of the chromosphere and corona is so tenuous that we cannot see it with our eyes except during a total eclipse, when we can see the faint visible light scattered by electrons in the corona [Section 2.5]. However, the roughly 10,000 K plasma of the chromosphere emits strongly in the ultraviolet, and the million K plasma of the corona is the source of virtually all X rays coming Magnetic fields trap gas. T 5,800 K sunspots T 4,500 K T 5,800 K convection cells Magnetic fields of sunspots suppress convection and prevent surrounding plasma from sliding sideways into sunspot. a Pairs of sunspots are connected by tightly wound magnetic field lines. Figure 15.17 Loops of magnetic field lines can arch high above the solar surface, reaching heights many times larger than Earth’s diameter. b This X-ray photo (from NASA’s TRACE mission) shows gas trapped within looped magnetic field lines. X-ray chapter 15 • Our Star 511
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: a This schematic diagram shows how
Page 19 and 20: Figure 15.20 An X-ray image of the
Page 21 and 22: e quator N N N magnetic field line
Page 23 and 24: THE BIG PICTURE Putting Chapter 15
Page 25 and 26: Problems 9. Gravitational Contracti

Chapter 15--Our Sun - Geological Sciences

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