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Airglow is the constant faint, amorphous near-infrared to near-ultraviolet radiation emitted by an atmosphere. This light is emitted by ions and molecules undergoing exothermic chemical reactions in the thermosphere of the Earth, or by individual atoms undergoing forbidden electronic transitions. The light emitted this way comes mainly from nitrogen and oxygen atoms, as well as by hydroxyl (OH-) radicals.

Airglow is an important consideration when performing astronomical photometry and spectroscopy. It is important in photometry, because it contributes to the noise of your observations. When you look at an astronomical source, you see not only light from the object, but any other light emitted or scattered along the line of sight. So when you compute the signal-to-noise ratio of your data, you have to add the photons from airglow to the Poisson errors of the source photons.

Airglow is important in spectroscopy because when you take a spectrum of a source, you will see bright emission lines superimposed upon the spectra of the source, due to light emitted by the air. These lines have wavelengths of:

  • 5199 Å (NI)
  • 5577 Å (OI)
  • 6300 Å (OI)
  • 6200, 6400, 6800, and 7200 Å vibration-rotation bandheads of OH

Thus when you want to measure the true spectra of your source, you must subtract these lines out, usually by measuring the spectrum of a blank patch of sky near your source.

There are also many man-made contributions to "night sky lines," though they cannot truly be called "airglow". These are primarily lines of sodium and mercury, which are scattered light from street lights. The brightest of these is the sodium D line, the color of the pale orange glow of low-pressure sodium lights. While natural airglow is an unavoidable component of astronomical observing (short of putting a telescope in space), man-made light pollution has become increasingly problematic for many observatories around the world.

I should also note that this glow is different from aurorae. Aurorae are caused by particles from the solar wind, directed by Earth's magnetic field into the atmosphere, where the atoms then fluoresce. Airglow does not require this external power source, and is much, much fainter. It is also spread out over the entire sky, unlike aurorae which come in sheets and waves.

Numbers obtained from http://sculptor.as.arizona.edu/foltz/nightsky/

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