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If I understand correctly, the H-alpha and Ca II K lines are absorption lines of the sun and allow to see the chromosphere. Following the Kirchhoff-Bunsen law, an absorption line is produced by a gas which is cooler than the source of light.

However the chromosphere is hotter than the photosphere and the temperature increases outward (https://www.researchgate.net/figure/The-figure-plots-profiles-of-mass-density-dashed-line-and-temperature-solid-line-in_fig6_312376685). This is in contradiction with the previous hypothesis. Then how can we see the chromosphere in absorption line? What am I missing in the reasoning? Is the Krichhoff-Bunsen law only valid for a thin medium and here we would be in presence of a thick one?

thank you in advance for your help.

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  • $\begingroup$ Are you sure you see absorption line spectra from the chromosphere? Can you link an example of such a spectrum? Probably you are mixing things up here. $\endgroup$
    – Py-ser
    Commented Apr 9, 2018 at 18:48
  • $\begingroup$ As I understand it, images of the sun in H-alpha (such en.wikipedia.org/wiki/File:HI6563_fulldisk.jpg ) are mainly in absorption line and show mostly the chromosphere. $\endgroup$ Commented Apr 9, 2018 at 20:55
  • $\begingroup$ You might be thinking about "filaments", which are generally cooler than much of the chromosphere. That's what shows up in the photos. $\endgroup$
    – Ken G
    Commented Apr 10, 2018 at 2:38
  • $\begingroup$ @sabik, nope. H\alpha pics of the Sun are definitely in emission: solarscience.msfc.nasa.gov/chromos.shtml $\endgroup$
    – Py-ser
    Commented Apr 10, 2018 at 17:24
  • $\begingroup$ As I understand it, the prominences are seen in H-alpha emission above the limb (such as during an eclipse) and in H-alpha absorption across the disk (and are then called filament). Indeed in this last case, the filaments are colder than their environment and I expect it to be in absorption (even if now understand that the chromosphere is in non-LTE regime and that I can not apply simply the Kirchoff-Bunsen law). $\endgroup$ Commented Apr 11, 2018 at 9:10

2 Answers 2

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Most of the absorption line comes from the photosphere in the region where the temperature is still dropping. Nevertheless, you can also get an absorption line where the temperature is rising, due to scattering effects. The Kirchoff-Bunsen law you describe is a simplified rule that assumes the lines form in thermal equilibrium with the local temperature, but sometimes scattering produces non-thermal-equilibrium ("non-LTE") effects. With scattering, line photons that get absorbed get re-radiated right away without coupling to the local temperature, and their re-emission has a 50% chance of being radiated back downward, where they will be reabsorbed deeper in the atmosphere. This contributes to absorption, especially near line center (and can cause a feature called "central reversal" there). But it is true that you can also get emission lines from very strong chromospheres where the Kirchoff-type effect does occur, see the profiles from chromospherically active dwarf stars (called "dMe stars").

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  • $\begingroup$ When light is absorbed and then re-emitted in a random direction, there is less than 0.0001% probability that it is directed at you. Since you did not detect the light, it appears as an absorption. $\endgroup$
    – LDC3
    Commented Apr 10, 2018 at 1:19
  • $\begingroup$ It's not quite that simple, because scattering will also take light that would not have come to you, and scatter it in your direction. If it's very optically thick, it can lose a lot of light, but if there's only an optical depth unity, it's more like a 50% reduction. $\endgroup$
    – Ken G
    Commented Apr 10, 2018 at 2:33
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I think this article summarizes really well.

You should not be confused about photosphere and beyond. From inside to photosphere, the temperature decreases with radius as a result of hydrodynamics; this seems natural. Beyond photosphere, there are chromosphere and corona. These layers are hotter than the photosphere, and the temperature increases with radius. We still do not know for certain why it is so. We can safely say that the plasma effects set in. This requires the better understanding of magnetohydrodynamics (MHD). Kirchhoff's law is invalid in this plasma regime.

Also, chromosphere and corona are seen mainly during the solar eclipse.

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