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ASTR 113 Lecture 2

ASTR 113 Lecture 2. with Prof. M. Opher Room 363 B Science and Technology 1 Telephone: 703-993-4571 FAX: 703-993-1269 E-mail: mopher@gmu.edu Office Hours: T/R 16:30-17:30 & 16:30-1. Photosphere- look like the Sun has a definite surface No! It’s a gaseous because its high temperatures!.

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ASTR 113 Lecture 2

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  1. ASTR 113Lecture 2 with Prof. M. Opher Room 363 B Science and Technology 1 Telephone: 703-993-4571 FAX: 703-993-1269 E-mail: mopher@gmu.edu Office Hours: T/R 16:30-17:30 & 16:30-1

  2. Photosphere- look like the Sun has a definite surface No! It’s a gaseous because its high temperatures! You can see only 400km into the photosphere

  3. What did we see last class? • Understand how the Sun produces energy. • Be able to explain hydrostatic equilibrium and thermal equilibrium. • Know what a stellar model is and be able to explain the theoretical model of the Sun. • Understand how conditions in the solar interior can be inferred from measurements of solar vibrations. • Know why scientists measure the number of neutrinos emitted from the Sun's core and what the results imply.

  4. The Photosphere -the lowest of three main layers in the Sun’s atmosphere • The Sun’s atmosphere has three main layers • the photosphere • the chromosphere • the corona Limb darkening: …when we look near the Sun’s limb we do not see as deeply into the photosphere Low T High T

  5. Absorption Lines in The Photosphere • Photosphere nearly a perfect blackbody with T=5800 K • *upper part of the photosphere is 4400K (cooler!)

  6. Granulation: convection cells ~ 1000 km (600 mi) The difference in T From center to edge Is ~ 300 K -> hot gas from lower levels rises upward, cools off and plunge back Into the Sun Form and disappear in few minutes.

  7. Supergranules Size: 35,000 km in diameter The convection moves at 1400km/h =900mi/h (Last a day!)

  8. More on… the Photosphere: • Opaque to visible light • 10-4kg/m3 (low density)

  9. The chromosphere is characterized by spikes of rising gas Using H filter • Above the photosphere is a layer of low dense (10-4 less than the photosphere) but higher temperature gases called the chromosphere • Emission Lines (in the red part of the spectrum-therefore the pinkish color)

  10. What about the Temperature?Its increases from 4400 K (top of Photosphere) to 25,000 K (top of chromosphere)

  11. Spicule: jet of rising gas Spicule last ~ 15 minutes; It roses 20km/s 300,000 spicules Exist at any one time Located at the edge of supergranules: (photosphere: descending gas and chromosphere: rising) ???? Answer: Magnetic Field

  12. Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules

  13. CORONA Extends for million of km’s. 10-6 brighter than The photosphere.. Streamers!

  14. CORONA • The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density • The solar corona blends into the solar wind at great distances from the Sun Emission lines In 530.3 nm (highly ionized Iron)

  15. The corona is actually not very hot: density is only 1011atoms/m3 While photosphere: 1023atoms/m3 Our air: 1025atoms/m3 ..if we will fly a spacecraft through the corona we Will have to worry with the heat from the photosphere!

  16. The corona ejects mass into space to form the solar wind Solar Wind: Bi-Modal Structure Property (1 AU) Slow Wind Fast Wind Flow Speed 400 km/s 750 km/s Density 7 cm-3 3 cm-3 Variance "large", >50% Variance "small", <50% Temperature T(proton, 1AU) ~ 200,000 K T(proton, 1 AU) ~ 50,000 K

  17. Activity in the corona includes coronal mass ejections and coronal holes

  18. Solar Minimum-Maximum

  19. Quiet Sun: Granules, supergranules, spicules, solar wind Active Sun: massive eruption; sunspots

  20. Sunspots are low-temperature regions inthe photosphere Sometimes they are isolated but frequently in sunspost groups ~ diameter of Earth (lasting between hours or months)

  21. T (umbra) = 4300 K T (penumbra) = 5000 K Stefan-Boltzman law : energy flux  T4

  22. Big group of Suspots (note The granulation In the undistorbed Sun)

  23. Galileo discovered that he could determine the Sun’s rotation rate by tracking Sunspots Carrington (1859) Showed that the Sun Does not rotate as a rigid body: equatorial regions rotate faster (27 days) Than the poles (35 days) Differential Rotation

  24. Sunspot Cycle: 11 years cycle

  25. Location of the sunsport vary: just after minimim the sunspots Are ~ 30 north and south of solar equator. • Why the sunspots should exist? Why they vary?

  26. Hale (1908) discover that the Sunspots are associated with intense magnetic field! Spectral lines are splitted: Zeeman Effect

  27. Magnetic Field Affects the Motion of Ionized Matter (Plasma)! Magnetogram: image of the Sun at two wavelengths: one just less and one just greater that the wavelength of the magnetically split spectral line. From the difference they can construct a picture Displaying the B of the sun. Blue is B (north polarity) and Yellow: B (south polarity) (like magnet)

  28. Sunspot group moving: the sunspots in front are called the “preceding members” the ones following behind are the “following members”. The preceding members in one solar hemisphere: same magnetic polarity; while the preceding members in the other hemisphere have the opposite polarity. Where the Sun has North Polarity the preceding members all have north magnetic polarity. In the opposite hemisphere, all the preceding members have south magnetic polarity.

  29. 22-Solar Cycle: the Sun reverses polarity every 11 years

  30. The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field (H. Babcock (1960))->differential rotation and convection

  31. More on that on page 398…

  32. Helioseismology: insights into the Sun’s magnetic field By comparing the speeds of sound waves in the interior ->rotation rate in the Sun’s interior. Pattern persist until the convection zone; In the radiative zone the Sun rotates as a rigid Body (27 days) The suspition is that B originate between the Radiative and convective Zone (when the two Slide past each other)

  33. Solar Variability can affect terrestrial climate During the Little Ice Age, London’s Thames River froze in winter, something that no longer happens. This 19th century engraving depicts the annual Frost Fair held on the ice-bound river, this one during the winter of 1683-84. Solar Variability Can Affect Terrestrial Climate Given the massive economic impact of small changes in climate, we should fully understand both natural and anthropogenic causes of global change.

  34. The Sun’s magnetic field also produces otherforms of solar activity • A solar flare is a brief eruption of hot, ionized gases from a sunspot group • A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona

  35. If we want to get to Mars: we need Space Weather

  36. TOPICS Know the structure of the solar atmosphere.Be able to describe the photosphere, its properties, and its features.Be able to describe the chromosphere, its properties, and its features.Be able to describe the corona, its properties, and its features.Know what sunspots are and what the sunspot cycle is.Be able to describe the magnetic-dynamo model of the sunspot cycle.Understand how activity in the solar magnetic field heats the corona.Know how solar eruptions affect us on Earth

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