Skip to content Skip to sidebar Skip to footer
Home / Which Atmispheric Later is Associated With the Suns Continuous Spectrum

Which Atmispheric Later is Associated With the Suns Continuous Spectrum

Post a Comment

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.

Back Next

The Photosphere

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

  • Limb Darkening:
    the edges are darker than the centre
  • Sunspots
Back Next

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
Back Next

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

Back Next

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.

Back Next

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.
Back Next

Movement of Sunspots

  • Movements of spots reveal that the Sun rotates with a period close to one month.
  • Equator rotates faster than the higher lattitudes. (Differential Rotation)
  • You can find photos of the Sun in many different wavelengths (updated daily) at the website: http://umbra.nascom.nasa.gov/images/latest.html

Back Next

Movie of Sunspot Motion http://solarscience.msfc.nasa.gov/surface.shtml

Movie Showing the motion of sunspots

Click on image for animation.

Back Next

Granules

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.)
Back Next

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").


Back Next

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
Back Next

A diagram of a spicule

Figure 18-15
Back Next

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.
Back Next


Prominences


 Back Next

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

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

mccorkledepud1987.blogspot.com

Source: https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect9/lecture9.html

Post a Comment for "Which Atmispheric Later is Associated With the Suns Continuous Spectrum"