Re: Solar absorption lines
- From: "George Dishman" <george@xxxxxxxxxxxxxxxxx>
- Date: Sun, 9 Jul 2006 17:07:15 +0100
"Scott" <ss@xxxxxxxxxxxxxxxx> wrote in message
news:44b10377$0$22123$afc38c87@xxxxxxxxxxxxxxxxxxxxxxx
Hi George,
Thanks for joining in this discussion.
Atoms in the chromosphere can scatter in all
directions but they are only illuminated from
one side, that facing the Sun.
Yes. But I don't see how that can create absorption lines.
If a patch of gas is illuminated by a source at
some brightness covering 2 pi steradians and then
emits the same energy into 4 pi sterardians it
must appear less bright.
If the photosphere completely surrounded a
patch of gas then you would be correct and
there would be a perfect balance between
absorption and emission,
Why can't there be a balance when the gas surrounds the emitter,
as in the case of the Sun?
The Sun doesn't surround a patch of gas in
the chromosphere, that's the key. (see below)
(Ignore situations where an atom
re-emits at a different wavelength, for the moment - that complicates
things.)
OK.
That's why they appear as emission when
viewing the chromosphere since we see
emission against a dark background.
Yes. I can understand why we see this during an eclipse. We're not
viewing the continuum (it's blocked by the moon), all we're seeing
is selected wavelengths of the continuum that are scattered by the
chromosphere.
As has been said, there are also other ways
for an excited atom to lose energy
You're refering to ionisation?
Thermal transfer in the gas, perhaps rotational
modes. I'm not sure what would be applicable in
detail.
but at
least part of the energy lost in the absorption
lines is being scattered.
I can see how individual photons can be scattered, but I don't see
how that can create an absorption line because, statistically, there
will be some other atom that will scatter an identical photon to
compensate for it.
Suppose a photon is scattered away from us:
(Sun) -> * Earth
|
v
You suggest another is scattered to replace it:
(Sun) * -> Earth
|
^
|
But the Sun is only on the left hand side, there
is no source at the bottom of the diagram so this
doesn't happen, the photon is not replaced. The
separations are not to scale, the * representing
the scattering atom would be almost on the surface
of the Sun at this scale but from the point of
view of the atom, the Sun fills half the sky, not
all of it, but the scattered photons go in all
directions.
Is that any clearer?
George
.
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