# absorption line from the chromosphere

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?

• 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. Apr 9, 2018 at 18:48
• 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. Apr 9, 2018 at 20:55
• You might be thinking about "filaments", which are generally cooler than much of the chromosphere. That's what shows up in the photos. Apr 10, 2018 at 2:38
• @sabik, nope. H\alpha pics of the Sun are definitely in emission: solarscience.msfc.nasa.gov/chromos.shtml Apr 10, 2018 at 17:24
• 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). Apr 11, 2018 at 9:10