Re: Solar absorption lines

Scott wrote:

1) the Doppler effect (photons can essentially not penetrate the solar
atmosphere within the line as the opacity here is so high; they only
get through once they have been shifted out of the line in the course
of the scatterings due to the frequency changes by the Doppler effect).

I assume this is a small effect (ie. (2) is the dominant effect) as we
do not see significant emission lines in the solar spectrum?

Well, the effect does not exactly result in emission lines, but merely
in a slight enhancement of the continuum near the absorption lines,
something like this:



1.4 *** ***
1 ****** * * ******* continuum level
* *
* *
* *
* *
* *
* *
0.2 ***


This effect must in principle necessarily occur. If you assume for
instance an infinite opacity within the line, no photon remaining
within the line will ever be able to penetrate the scattering layer. It
can only reach the observer once its frequency has been shifted into
the region where the opacity is small (i.e. outside the line).
So this mechanism would actually produce absorption lines without that
any photons are lost. However, it is likely that the photons within the
line are already destroyed before this happens, e.g. due to
photoionization of excited levels, so in this sense it might not be
significant.



2) Photoionization (photons within the line are trapped for so long in
the solar atmosphere that they have a high probability of ionizing
excited states of hydrogen; this means they are lost from the line)

What happens to these electrons after they're ionised?

In your previous post in this thread, you said:

"The photoionization process leads then
subsequently again to a photon on recombination of the photoelectron,
but this will have a completely different wavelength"

& Martin said that the electrons will "radiate at a frequency dependant
on their speed and the field strength".

So wouldn't we be seeing emission lines in the solar spectrum for these
2 effects? (radiating & recombination)

Yes, when the electrons recombine, they produce emission lines, but not
necessarily at the wavelength of the absorption lines. If you assume
for instance that an H-alpha photon (656 nm) photoionizes an excited
atom which is in the n=3 state, and then assume that the photoelectron
produced this way recombines into the n=2 state, this will lead to a
photon at 328 nm (and then a further photon at 122 nm when the atom
decays into the ground state n=1). So the original photon within the
656 nm line will be lost for good in this case. If the recombination
goes into the n=3 state and then subsequently into the n=2 state, the
656 nm photon is of course recovered, but this is only the case for a
fraction of the recombination events. The situation is even worse for
absorption lines other than hydrogen, because as hydrogen is by far the
dominant element, electrons will in all likelihood recombine with
hydrogen ions (protons) rather than with ions of the minor elements. So
for the latter, the losses within the absorption lines due to
photoionization will be close to 100% as subsequent recombination
produces radiation at the hydrogen frequencies rather than at the
frequencies of the element considered.

Thomas

.

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