Lecture 9: The Sun's Photosphere and Chromosphere

Chapter 16: 8th Ed. pages 414-428 or 3rd Ed. pages ???

The Sun

The Sun can be split into two regions:
  • The interior is a sphere with radius R = 7x108m
  • The atmosphere lies on top and has the following layers (from innermost to outermost):
    • The photosphere is about 300 km thick. Most of the Sun's visible light that we see originates from this region.
    • The chromosphere is about 2000 km thick. We only see this layer and the other outer layers during an eclipse.
    • The corona extends outwards for more than a solar radius.
The size of the atmosphere compared to the Earth.
Diagram showing the Sun's atmosphere
and the Earth to scale.

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The Photosphere

A Photo of the Sun's Photosphere shows the following features:

  • Limb Darkening:
    the edges are darker than the centre
  • Sunspots
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Limb Darkening: (An exaggerated diagram of the photosphere)

  • We see light originating from a (almost) constant distance from the top of the photosphere.
  • Near the middle of the Sun's disk, we see deeper into the photosphere than at the edge of the disk. (Edge = Limb)
Limb darkening
  • Limb darkening is evidence that the temperature of the Sun's photosphere decreases outwards.
    • Luminosity of a blackbody depends on Temperature.
    • Hotter inner layer will be brighter than the cooler outer layer.
  • Temperature at bottom of photosphere: 6400 K
  • Temperature at top of photosphere: 4600 K
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The Sun's Spectrum is an Absorption Spectrum

  • Since the photosphere is cooler but less dense than the interior region it is the screen that allows the continuous blackbody spectrum to be seen through.
  • Only at the wavelengths at which atoms in the photosphere can absorb light will photons be impeded in their outward travel.
  • The result is an absorption spectrum, a continuous blackbody spectrum with dark absorption lines superimposed on it.
  • The fact that we see an absorption spectrum when we look at the photosphere is evidence that the temperature of the photosphere decreases outwards.
Figure 13.5

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Sunspots: (Photo of the Sun taken on Jan. 17, 2005)

  • Sunspots are regions with high magnetic fields (1000 x higher magnetic field than average!)
  • Typical size of spots is similar to the size of the Earth.
  • These regions are cooler than average, so they look darker than the surrounding hotter region.
  • Sunspots are related to X-ray flares, mass ejections and the aurora seen on Earth.

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Close-up Picture of a group of Sunspots

  • The darkest regions (umbra) have the largest magnetic fields and the coolest temperatures. The outer brighter region is the penumbra.
  • Sunspots come in pairs: each member of the pair has opposite polarity. (I.e. one is a north magnetic pole, the other is south.)
  • Each sunspot region lasts for a few days to a few weeks.
  • The filaments in the penumbra are due to the magnetic lines of force.

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Movement of Sunspots

Jan. 16 photo Jan. 17 photo
Jan. 16, 2005 Jan. 17, 2005
  • Movements of spots reveal that the Sun rotates with a period close to one month.
  • Equator rotates faster than the higher lattitudes. (Differential Rotation)

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Movie of Sunspot Motion http://solarscience.msfc.nasa.gov/surface.shtml
Movie Showing the motion of sunspots

Click on image for animation.

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Similar to Figure 18-11 Figure 18-12

Close-up Picture of the Photosphere

  • Granules are the cell-like features seen on the Sun's photosphere.
  • The granules are the tops of convective cells which lie in the convective zone just below the photosphere.
  • Each cell ranges in size from 100 km to 1000 km across and may last up to half an hour ( dynamical time scale ! ).
  • The bright regions are zones where hot gas rises.
  • The dark borders are the places where the cool gas sinks.
  • The gas moves outwards or inwards at speeds up to 7 km/s = 25,000 km/hour. (Measured through Doppler shifts.)
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The Sun's Chromosphere

The Sun During an Eclipse

Time-lapse photos of an eclipse

A Solar Eclipse

  • The photosphere is much brighter than the outer parts of the Sun's atmosphere (the chromosphere and the corona), so regular photos of the Sun do not show the outer atmosphere.
  • During a solar eclipse the Moon blocks out the light from the photosphere and we can only see the light coming from the chromosphere and corona.
  • This series of eclipse photos show successively longer exposures so that fainter details can be seen.
  • In the first picture we can only see the chromosphere ("coloured sphere") which shows up a a few red features at the edge of the Sun.
  • In the longer exposure pictures we can see the fainter Corona ("crown").

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The Chromosphere with a close-up of the spicules.

  • The Chromosphere is not exactly a sphere: there are many spicules and prominences which jut outwards.
  • Magnetic fields help support the spicules and the prominences.
  • The red colour results from the emission of Balmer-alpha photons : electrons jumping from the n=3 level to the n=2 level.
  • The emission lines can only occur if the gas in the chromosphere is very hot and the density is very low. And we do not look straight through ! The chromosphere is hotter (but less dense) than the photosphere!
Figure 18-14

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A diagram of a spicule

Figure 18-15
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Variation of Temperature in the Sun's Atmosphere

See Figure 13.4
  • Photosphere: Temperature decreases outwards.
    • At bottom: T = 6400 K
    • At top: T = 4000 K
  • Chromosphere: Temperature increases outwards.
    • At top: T = 10,000 K
  • Transition Zone: Temperature shoots up to near 1 million K
  • Corona: Temperatures increase to about 2 million K
  • The source of this heat is not well understood. Current theories suggest that magnetic waves might transport energy from the convective zone to the corona.

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  • Close-up picture of the chromosphere showing a prominence.
  • The prominences are loops of gas which arch over sunspot regions.
Figure 13.12

Quiescent Prominences in the Chromosphere

  • A close-up picture of the prominences (loops).
  • The dark lines are filaments, which are the same thing as prominences.
  • The quiescent prominences are very stable and can last weeks or months.

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Eruptive Prominences

  • Some of the prominences will erupt, causing gas to be flung outwards.
  • In this picture the gas travels outwards about 70,000 km in the course of a few hours.
  • Prominences are more likely to erupt when the magnetic fields near the sunspots are changing.
Figure 13.13

Next lecture: The Corona
Read Chapter 16, 8th Ed. pages 414 - 428 or 3rd Ed. pages ???.