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The Photosphere

The Photosphere. The point at which gas is thick enough that we can’t see through it This is a little shallower towards the edge Limb darkening. Here we see shallow and cool. Here we see deep and hot. Granulation. The Sun’s surface is not uniformly bright Hot regions of upwelling gas

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The Photosphere

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  1. The Photosphere • The point at which gas is thick enough that we can’t see through it • This is a little shallower towards the edge • Limb darkening Here we see shallow and cool Here we see deep and hot

  2. Granulation • The Sun’s surface is not uniformly bright • Hot regions of upwelling gas • Cooler regions of falling gas • Convection near the surface https://www.youtube.com/watch?v=W_Scoj4HqCQ

  3. Granulation • Granulation changes over time

  4. Granulation • Circulation of gases in outer layer • Hot plasma rising, cool plasma falling – convection • Underneath, evidence of circulation on a larger scale • Supergranules larger than the Earth Earth

  5. Chromosphere • A cooler layer just outside the photosphere • Light from photosphere lines absorbed from this layer • Bright line spectrum • Visible during eclipses • Red color from spectral line of hydrogen

  6. Spicules • Bright small jets that reach from the photosphere up through the chromosphere • Last about 5 minutes • Cause not understood • Thousands on the Sun at any given time

  7. Corona: Temperature • Thin, extremely hot region • Visible during eclipses • High temperature not well understood • Heated by magnetic fields • Like a whip • Low density – has little total heat Q. 63: Coronal Spectrum

  8. Sunspots • Relatively cool regions on a brighter, hotter background • Large magnetic fields What causes them: • Magnetic field sticks out • Plasma can’t flow horizontally • Prevents convection from working • Hot gas can’t rise • Temperature cooler

  9. Sunspots - Structure Penumbra Umbra

  10. Sunspots Often Come in Groups • Pairs are common • North with south • Larger groups are not uncommon

  11. The Solar Cycle • The number of sunspots changes over time • Approximately an eleven year cycle: Butterfly diagram • Sunspots first appear far from the equator • Over time, they become more numerous, and move towards the equator • Then they all disappear • Each cycle of eleven years, the Sun’s north and south magnetic poles switch Q. 64: Sunspot Cycles

  12. The Solar Cycle • Sun’s North/South magnetic pole reverses each cycle • Not all cycles are created equal! • 1650-1700, very low level, “Maunder Minimum” • Corresponded to “Little Ice Age” in Europe • Level of sunspots/magnetic activityaffects all othersolar activity

  13. The Solar Cycle

  14. Flares Explosive release of magnetic energy on surface of the Sun • Typically reach through photosphere, chromosphere, to corona • Burst of energy, especially X-rays • Can cause coronal mass ejections

  15. Prominences Loops of magnetic fields sticking high above the Sun’s surface

  16. Prominences

  17. Prominences

  18. Prominences • Chargedparticlesfollowingmagnetic linesfrom insidethe Sun • https://www.youtube.com/watch?v=EATDvxnLXyc

  19. Coronal Mass Ejections • Flares can expel gas at high velocities from the Sun • Similarly, prominences can become “disconnected” • High temperature gas expelled at high velocity • Up to half the speed of light • These events are called coronal mass ejections • They can cause satellite malfunction and communication disruptions on Earth • They pose a major danger to astronauts beyond the Earth’s magnetosphere

  20. Coronal Holes • Most gas in the corona is trapped by the Sun’s magnetic fields • “Coronal holes” allow some to escape to space • This creates the solar wind

  21. The Solar Wind • Particles flowing outward from Sun travel to Earth and beyond • Contains hydrogen, helium, etc. • Magnetic fields Earth funnel them to Earth’s magnetic poles • Causes the Aurora Borealis and Aurora Australis

  22. Helioseismology • Vibrations of the Sun tell us some things about its interior • Much like earthquakes for the Earth • Need to be matched with detailed computer models of interior of Sun • Together with our knowledge of its composition and total power output

  23. Interior Structure of the Sun There are three layers to the interior of the Sun • The Core: • Where the energy is generated • The Radiative zone • Heat is transported by radiation • The Convective zone • Heat is transported by circulation

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