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Exploring the Sun: Structure, Sunspots, and Solar Activities

This chapter delves into the sun, our closest star, located just 8 light minutes away. It covers key solar properties including rotation, structure (core, radiation zone, convection zone, photosphere, and more), and solar luminosity. The sun's complex features like sunspots, their cooler temperatures compared to surrounding areas, and their significant magnetic fields are discussed. We also explore solar phenomena such as solar wind, solar granulation, and the role of the Solar and Heliospheric Observatory (SOHO) in understanding solar dynamics.

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Exploring the Sun: Structure, Sunspots, and Solar Activities

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  1. Chapter 9The Sun

  2. Sunspots

  3. Our Sun • Is the nearest star • 8 light minutes away • Next nearest star is 4.3 light-years away (300,000X further than sun)

  4. Figure 9.1The Sun

  5. Table 9.1Some Solar Properties

  6. Solar Rotation • Differential rotation • Measure by timing sunspots • Faster at equator • Slower at poles • X-ray and visible light movie of sun

  7. Figure 9.2Solar Structure

  8. Solar Structure • Core • Radiation zone • Convection zone • Photosphere (“surface” we see) • Chromosphere • Transition zone • Corona • Solar wind

  9. Solar Luminosity • Solar constant 1400 W/m2 above earth’s atmosphere • 1000 W/m2 at earth’s surface • Solar constant and distance to sun gives luminosity of about 4 X 1026 W

  10. Figure 9.3Solar Luminosity

  11. Stellar balance • Outward pressure of hot gas • Inward pull of gravity • Balanced at every point in a star

  12. Figure 9.4Stellar Balance

  13. Standard solar model • Mathematical and physical model of sun • Based on observations and physical laws • Predicts density and temperature • Helioseismology allows knowledge of interior structure

  14. Figure 9.5Solar Oscillations

  15. Figure 9.6Solar Interior

  16. Sun’s temperature and density • Core density 150,000 kg/m3 (20X iron) • Core temperature 15 million K • Core is a gas (plasma) • Photosphere is 0.0001X density of earth’s atmosphere • Photosphere temperature 5780 K

  17. Discovery 9.1SOHO: Eavesdropping on the Sun

  18. SOHO • Solar and Heliospheric Observatory • European Space Agency

  19. Solar energy transport • Near core - very hot • Gas is completely ionized (plasma) • No photons captured - transparent to radiation • Outer edge of radiation zone cool enough for electrons to re-combine with nuclei • Photons all absorbed

  20. Convection Zone • Energy transported by rising hot gases • Cooler gas sinks • Convection cells vary in size with depth in convection zone • Tens of thousands of km to a thousand km convection cells • Photons from photosphere escape into space

  21. Figure 9.7Solar Convection

  22. Evidence for convection • Solar granulation of photosphere • Granules size of a large US state • Last 5 to 10 minutes • Bright regions - hot gas rising • Dark regions - cool gas sinking • 500 K difference between hot and cool

  23. Figure 9.8Solar Granulation

  24. Solar Granulation movie • Near infrared • 60 minute sequence sped up • At http://www.bbso.njit.edu or click here

  25. Doppler shift • Bright granules move up at about 1 km/s • Dark granules move down at about 1 km/s

  26. Supergranulation • Larger scale flow beneath solar surface • 30,000 km across

  27. Figure 9.9Solar Spectrum

  28. Composition of solar atmosphere • Primarily H and He • Also O, C, N, Si, Mg, Ne, Fe, S • Similar to Jovian planets and rest of universe

  29. Table 9.2The Composition of the Sun

  30. Figure 9.10Solar Chromosphere

  31. Solar chromosphere • Above photosphere and less dense • Pinkish hue from H emission • Expelling jets of hot matter - spicules • Last minutes • 100 km/s

  32. Figure 9.11Solar Spicules

  33. Corona • Corona visible during total solar eclipse • Emission spectrum visible against blackness of space • Ionized atoms - high coronal temperatures

  34. Figure 9.12Solar Corona

  35. Transition Zone • Minimum temperature of 4500 K in chromosphere • Temperature climbs through transition zone • Reaches several million K in corona

  36. Figure 9.13Solar Atmospheric Temperature

  37. Solar wind • Starts 10 million km above photosphere • Hot coronal gas escapes sun’s gravity • Millions of tons of sun ejected each second • Only lost 0.1% of mass in 4.6 billion years

  38. X-rays in corona • Photosphere emits primarily visible light • Hotter corona emits primarily X-rays • Coronal holes - visible in X-rays • Solar wind escapes in coronal holes • Related to magnetic fields

  39. Figure 9.14Sunspots

  40. Figure 9.15Sunspots, Up Close

  41. Sunspots • In photosphere • Cooler (darker) than surrounding material • Dark umbra (4500 K) • Grayish penumbra (5500 K) • Typically 10,000 km across (size of earth)

  42. Sunspot magnetism • Magnetic field of photosphere stronger than earth’s • Magnetic field in sunspots is 1000X greater than surrounding photosphere • Field lines perpendicular to surface • Strong fields interfere with convective flow • Causes sunspots to be cooler

  43. Sunspot magnetic polarity • Sunspots in pairs at same latitude • Pair members have opposite polarity N&S • Leading spot in a hemisphere always has same polarity • Leading spot in other hemisphere has opposite polarity

  44. Figure 9.16Sunspot Magnetism

  45. Magnetic field wrapping • Differential rotation “wraps” magnetic field • North-south re-oriented to east-west • Convection lifts field to surface • Twisting and tangling results • Some kinks rise out of photosphere • Forms sunspot pair

  46. Figure 9.17Solar Rotation

  47. Analogy 9.1A tangled garden hose

  48. Figure 9.18Sunspot Cycle

  49. Maunder minimum • Cycle varies from 7 to 15 years • Overall activity varies • Solar inactivity from 1645-1715 • Maunder minimum caused “Little Ice Age”

  50. Figure 9.19Maunder Minimum

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