Why Does Spectral Line Darkening Occur Despite Photon Re-Emission?

[Joe Prendergast]

Suppose a photon from the sun's photosphere, initially traveling toward earth, is absorbed by an atom in the sun's chromosphere. The electron then transitions to its first excited state and spectral darkening is observed at a distinct wavelength on earth. I've read that the electron only stays in its excited state for a brief instant. When the electron transitions back to its ground state it emits a photon with the same wavelength where the spectral darkening had occurred. Why would this not get rid of the darkening seen from earth? Is it because the emitted photon travels in a random direction (not necessarily toward earth)?

 

[Bandersnatch]

Is it because the emitted photon travels in a random direction (not necessarily toward earth)?

Yes, that's correct.

 

[Ken G]

That may be true in the low density chromosphere, but there is more to the question of "line darkening" (usually called absorption lines, or "Fraunhoefer lines") because most of those lines are formed in the deep photosphere where the density is very high. In very high density conditions, often when an atom absorbs a photon it experiences a collision with another particle that removes that excitation energy before the photon can be reemitted. This "absorbs" the photon instead of "scattering" it, in which case the energy shows up somewhere else in the continuum spectrum of H minus opacity, rather than in the line. So there it is not the direction the photon is traveling that gets changed, it is the wavelength where the energy eventually emerges.

And even when it is scattering that is causing the absorption line, the change in direction alone could not produce such a darkening, because the Sun is a sphere so making the light come out at a different angle means it is still seen somewhere outside the Sun, so that wavelength photon would eventually be seen coming from somewhere on the Sun by somebody (and the Earth is not in a special position to see anything different). To get a true darkening still requires absorption, it's just that if you bounce the light back downward toward the Sun, it has a good chance of being absorbed by non scattering processes before it gets out again. So absorption lines are typically caused by a combination of scattering that changes the direction and photon destruction that moves the energy out of the line bandwidth.