# How deep is the "A" Fraunhofer line in the solar spectrum? Is it from the Sun's or Earth's atmosphere?

Question: Where can I see - or even download - a fairly high resolution (~0.1 nm) solar spectrum at zero air mass - in other words from space without absorption and other features from the Earth's atmosphere? Of particular interest is the near infrared around 750-780nm. Is the "A" Fraunhofer line due to absorption by oxygen in the Sun's atmosphere, or in Earth's?

The background discussion below shows some examples, but these are not directly from scientific sources and I have a hunch they are from observations on Earth's surface below the atmosphere.

Background:

Studies of photosynthesis on Earth using spectrometers in satellites measure solar flux induced fluorescence from foliage during the day, taking advantage of certain bands in the solar spectrum that are relatively "dark". GOME-2

For example, the Fraunhofer line labeled "A" below is produced by absorption by oxygen, but it is not clear to me if it is the oxygen in the Suns atmosphere or the Earth's atmosphere that's producing this line.

The images below are spectra of sunlight with various instruments of widely different resolutions, and because the absorption is narrow, the apparent depth is a function of resolution rather than reality.

above: a low resolution spectrum from here

above: An spectrogram of the Sun from this NOAO website (N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF) with the "A" line near 760nm appearing near the top, a little right of center. The original data is from the "Fourier Transform Spectrometer at the McMath-Pierce Solar Facility at the National Solar Observatory on Kitt Peak, near Tucson, Arizona" but seems to be formatted and colorized to look like an Echelle spectrogram.

A video describing the use of the dark band for Earth science can be seen at https://youtu.be/1XilneV3cJI.

Where can I see - or even download - a fairly high resolution (~0.1 nm) solar spectrum at zero air mass - in other words from space without absorption and other features from the Earth's atmosphere?

You can download a normalized solar spectrum with 0.1 nm resolution from here. However this is not at zero airmass.

A solar spectrum with zero airmass can be found on this website with the direct link to their data in an excel sheet here. This spectrum appears to be at a 1 nm resolution.

Of particular interest is the near infrared around 750-780nm. Is the "A" Fraunhofer line due to absorption by oxygen in the Sun's atmosphere, or in Earth's?

If you look up the Fraunhofer A line (really it's a "band"), you'll see it is caused by molecular oxygen, namely $\mathrm{O_2}$. This should tell you right away that it is caused by our atmosphere as molecules do not easily exist in the atmosphere of the Sun. In our Sun's atmosphere, most species are ionized forms of single atoms such as $\mathrm{O\:IV}$.

In Astronomy, spectral lines which are caused by the Earth's atmosphere and not by the source object being observed, are referred to as Telluric lines (from the latin word tellus meaning Earth). I found this source which describes, in detail, the various telluric lines that you may encounter.

• Well it didn't tell me right away, but that's just me. :-) Thank you for the great links and explanations! This is exactly what I needed. Wow! I was wondering what Tellurium was doing in the sun, but your explanation cleared that up as well.
– uhoh
Aug 24, 2016 at 13:51
• @uhoh I didn't mean to suggest that you should have known, simply that you can know based on the fact that the A line is caused by a molecule. But now at least you'll know for the future. Aug 24, 2016 at 14:12
• Actually, some of the absorption lines in the solar spectrum are from molecules (e.g., CN, CO, OH), though these are mostly in the IR and in cooler regions like sunspots. But you're certainly correct about O_2. Aug 24, 2016 at 14:23
• Oh I understand - I was trying to be cute. I have to remember that humor in ASCII in an international setting doesn't always work. I once used "so-and-so is asking for the Moon" meaning asking for too much, but of course in Astronomy SE that one's got a high probability of missing its mark. Thank you again for a great answer!
– uhoh
Aug 24, 2016 at 14:25
• @PeterErwin Yes, that is true. I suppose it was bad to say such a blanket statement that no molecules exist in the Sun. Aug 24, 2016 at 14:25