Solar absorption lines

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.

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

On Sat, 08 Jul 2006 00:15:26 +1000, Scott wrote:

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

The emission lines are there. The continuous spectrum is produced by
the photosphere and the lines are produced by the chromosphere. When
there is an eclipse of the sun a spectroscope aimed at the limb of the
sun will show a flash spectrum of emission lines in that brief period
of time when the moon hides the photosphere but exposes the
chromosphere. The emission lines are at the same wavelengths as the
absorption lines.

Thomas Smid wrote:

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).

Scattering does not create a net bias where photons can be scattered
away from an observer. If this were true, then the spectra recorded
from the sun would vary depending on your vantage point.

Scott.

William Hamblen wrote:
On Sat, 08 Jul 2006 00:15:26 +1000, Scott wrote:

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

The emission lines are there.

Yes, but they are dominated by absorption lines. There is much more
absorption than emission.

The continuous spectrum is produced by
the photosphere and the lines are produced by the chromosphere.

I would like to understand how the chromosphere produces absorption lines.

When
there is an eclipse of the sun a spectroscope aimed at the limb of the
sun will show a flash spectrum of emission lines in that brief period
of time when the moon hides the photosphere but exposes the
chromosphere. The emission lines are at the same wavelengths as the
absorption lines.

I'm not sure what you mean by "flash spectrum". Are you refering to
solar flares? (which cause emission lines to be convolved with the
standard solar spectrum)

Scott.

On Sat, 08 Jul 2006 20:22:50 +1000, Scott wrote:

William Hamblen wrote:
On Sat, 08 Jul 2006 00:15:26 +1000, Scott wrote:

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

The emission lines are there.

Yes, but they are dominated by absorption lines. There is much more
absorption than emission.

The continuous spectrum is produced by
the photosphere and the lines are produced by the chromosphere.

I would like to understand how the chromosphere produces absorption lines.

When
there is an eclipse of the sun a spectroscope aimed at the limb of the
sun will show a flash spectrum of emission lines in that brief period
of time when the moon hides the photosphere but exposes the
chromosphere. The emission lines are at the same wavelengths as the
absorption lines.

I'm not sure what you mean by "flash spectrum". Are you refering to
solar flares? (which cause emission lines to be convolved with the
standard solar spectrum)

The light emitted by a hot, dense substance forms a continuous
spectrum. The region of the sun that is made of a hot plasma and
emits a continuous spectrum is the photosphere. When light passes
through a thin, ionized gas selected wavelengths are absorbed in
narrow bands. Thin, ionized gases also emit light. The spectrum is
not continuous but consists of specific wavelengths that depend on the
chemical element involved and the degree of ionization. Each element
has a characteristic spectrum and for a given element the absorption
lines and emission lines are at the same wavelength. This was
discovered in the laboratory in the 19th century and made it possible
to identify the elements in the outer parts of the sun. The outer
region of the sun that is made of a thin, ionized gas, and produces
the numerous, narrow black lines visible in the spectrum of the sun,
is the chromosphere. During a solar eclipse the chromosphere is
visible to the eye as a red line at the edge of the sun. When the
chromosphere, but not the photosphere, is briefly visible during the
eclipse the flash spectrum consists of emission lines that are at the
same wavelengths as the absorption lines usually visible in the solar
spectrum. It is called the flash spectrum because it is visible only
as a brief flash during the eclipse. You could do a Google images
search for "flash spectrum" to see examples on the world wide web.

An electron absorbs and emits radiation at wavelengths depending the
changes in energy of the electron. Electrons in atoms can have only
certain energies. Because of this, under the right conditions, you
see emission or absorption lines in the spectrum. Calculating the
energies and wavelengths in a spectrum is part of modern physics.

Read some basic books on astronomy and physics. One of the
astronomy-for-non-science-majors texts such as George O. Abell's
Exploration of the Universe is a good choice for the astronomy book.
Consult a dictionary for the meaning of convolve while you are at it.

Bud

Hi William,

Thanks for your reply.

The light emitted by a hot, dense substance forms a continuous
spectrum.

You give an excellent description of the concepts required to
understand a spectrum. Alas, I already understand all this
(though I wasn't familiar with the term "flash spectrum") &
my question is still not answered.

The fault is mine, however. I'm finding it difficult to formulate
my problem in words.

I'm familiar with quantisation of photons & energy levels of atoms
& how atoms can absorb & re-emit photons, etc.

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.)

Consult a dictionary for the meaning of convolve while you are at it.

I think my usage is correct. It is a derivative of the technical term
"convolution". See: http://en.wikipedia.org/wiki/Convolve

Though, I agree, colloquial use of the term means something different.

Scott.

Scott wrote:
Thomas Smid wrote:

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).

Scattering does not create a net bias where photons can be scattered
away from an observer. If this were true, then the spectra recorded
from the sun would vary depending on your vantage point.

Scott.

Imagine that you have a light bulb around which you wrap some highly
reflective material (e.g. an aluminium foil). Now what does this do to
the brightness of the light bulb? It will obviously be drastically
reduced.
The solar absorption lines are essentially produced by the same effect
because for certain wavelengths (and only for these) the upper
photosphere and chromosphere effectively act as reflecting layers.

Thomas

On Sat, 08 Jul 2006 23:38:55 +1000, Scott wrote:

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).

Your question is really "why are there ever absorption lines?" If
atoms also emit the light they absorb, why do we see absorption lines
at all? This would apply to the laboratory as well as to stars. Part
of the reason is that direction of the emitted radiation has no
relationship to the direction of the absorbed radiation. Part of the
reason is that the electrons have many possible energy levels and
don't have to emit radiation at the same wavelength they absorbed it -
they could make one jump up, but two jumps down, for example.
Electrons also can become unbound from atoms and radiate freely.

"Scott" wrote in message
u...
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