Re: Solar absorption lines
- From: "Thomas Smid" <thomas.smid@xxxxxxxxx>
- Date: 7 Jul 2006 09:03:06 -0700
Scott wrote:
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.
Well, the point is that the radiation field in the solar atmosphere
*isn't* isotropic (as you seem to be suggesting), so the net effect of
scattering is not zero (after all, the radiation has a net propagation
direction *away* from the sun).
Thomas
.
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