Hi all,

What process causes absorption lines in solar spectra (when
measured from above the Earth's atmosphere)?

I understand how absorption lines can occur in _extra_-solar
spectra due to an intervening cloud of interstellar gas/dust. The
atoms in the cloud can absorb part of the (effectively blackbody)
continuum (depending on the energy level of its electrons) & then
re-emit it in a random direction, away from the observer.

However, in the case of the Sun [1] the "cloud" (actually the upper
atmosphere of the Sun) completely surrounds the Sun so the _net_
effect of scattering is zero - for any photon heading toward an
observer that is scattered _away_, there is another photon heading
away from the observer that is scattered _toward_ them, statistically
speaking.

I thought, perhaps, that the cascade effect (photons being
absorbed & then re-emitted, potentially at different wavelengths)
could explain the absorption lines. But then I would expect to see
_emission_ lines on top of the continuum - if there's a reduction
of H-alpha, say, then there must be an abundance of some other
wavelength(s). (ie. conservation of energy)

But if I look at solar spectra, I don't see any significant
emission lines convolved with the continuum.

Any help to align my understanding with reality would be muchly
appreciated.

Scott
[1] actually this occurs in all stars, of course.