Re: Solar absorption lines


"Scott" <ss@xxxxxxxxxxxxxxxx> wrote in message
news:44afb581$0$22358$afc38c87@xxxxxxxxxxxxxxxxxxxxxxx
What I'm trying to understand is _what_ causes the absorption lines.
Obviously, atoms in the chromosphere can absorb specific wavelengths
of the continuum. BUT, they're not absorbed forever. Indeed, a fraction
of a second later the atom re-emits a photon (and as you point out the
"flash spectrum" is proof of this).

& _unlike_ interstellar gas/dust which can scatter photons from
extra-solar stars away from an observer, the chromosphere can't
scatter photons away from the observer. (Well, as I describe
earlier in this thread, individual photons can be scattered away,
but they're compensated for by other atoms scattering photons
_toward_ the observer.)

Atoms in the chromosphere can scatter in all
directions but they are only illuminated from
one side, that facing the Sun.

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, which of course is
why black body radiation is independent of
the material.

That's why they appear as emission when
viewing the chromosphere since we see
emission against a dark background.

As has been said, there are also other ways
for an excited atom to lose energy but at
least part of the energy lost in the absorption
lines is being scattered.

Does that help?

George


.