Airglow
Published on Nov 17, 2006 at 2:11 pm.
3 Comments.
Filed under Earth, astronomy, atmosphere, skywatching.
Yesterday, I talked about twilight. I made it sound like once all the twilight was gone, then it got totally dark, with no light except that from the stars (moon, planets, streetlights, etc). Well, that isn’t quite true. As it turns out, it never gets completely dark. There is always some sky glow. And, that sky glow is not always due to light pollution. Most annoyingly, the atmosphere itself glows! So, the sky can never be fully dark!
This glowing of the air is called airglow. It is readily measurable from the ground. Airglow is also probably far brighter than most people would think. If you get far away from any artificial lights, you can even see it with the naked eye! Overall, airglow accounts for more light in the sky on totally dark nights than does starlight. It is normally brightest about 10 to 20 degrees from the horizon (The lower that you look, the more air that you look through, but if you look too low, the light from distant parts of the atmosphere is scattered too much. A ballance comes about 15 degrees altitude.) In fact, airglow was first discribe scientifically by Anders Angstrom in 1868. You can see airglow from space, too. The photograph at the top of this post was taken by Don Pettit from aboard the International Space Station. Orion is seen just above the limb of the Earth. The colored arc across the bottom of the photo is caused by airglow. For those familiar with Orion, look carefully at Rigel (the lower right star of Orion). You can see the distortion of that star’s image as it is seen through the atmosphere.
There are several sources of airglow. That really shouldn’t be surprising, since there are a number of things that can affect the atmosphere. Most of the sources of airglow, though, can be traced back to the Sun.
Ultraviolet light from the Sun can excite atoms in the upper atmosphere. In fact, these atoms can even be ionized by the ultraviolet light. As the ionized atoms recombine, they emit light. Reds and greens are given off by oxygen and nitrogen atoms. A yellow-orange color is given off by sodium atoms about 55 to 60 miles up. This, in fact, is one of the larger contributors to the airglow, even though sodium is a very minor component of the atmosphere. Some of the atoms absorb visual or ultraviolet light, exciting them to higher energies. They then de-excite, giving off this same light. Sometimes, though, they collide with one another, giving up some of their energy mechanically. They can then give off light with lower energy than they absorbed (this is called fluorescence). So, the airglow is not limited to just certain spectral colors of light. Much of the light during astronomical twilight results from this excitation of the upper atmosphere. Obviously, there is more airglow during the daytime, but scattered light from the Sun makes it impossible to see. After the Sun sets on the upper atmosphere, there is no more photoexcitation or photoionization going on, but there are still plenty of excited and ionized atoms giving off light to last through the night.
Another source of light is through chemiluminescence. Here, the ionized atoms interact chemically with other substances in the air, such as hydroxyls. The chemical reactions give off light. Again, sunlight is the primary souce of the ionized atoms to begin with.
But, even without the Sun, the air would glow. Cosmic rays slam into the atmosphere. Cosmic rays can come in the form of ultra high energy gamma rays or ultra fast moving particles. Both have a similar effect. They ionize atoms, and then the same things happen that would happen when sunlight ionizes atoms.
Another source of airglow comes from some of the ultrahigh energy cosmic gamma rays. High in the atmosphere, they undergo a pair production event. Einstein’s famous equation E=mc2 says that mass and energy are interchangable. In my post on what is going on inside the Sun a while ago, I mentioned that some of the matter in the Sun is turned into energy. Well, the reverse can also happen. Energy can become a particle and an antiparticle. If the initial gamma ray has enough energy, these particles can themselves have enough energy to be moving very fast, particularly if they are light, such as an electron and a positron pair. Sometimes, these particles can even be going faster than light in the air. (Note: nothing can go faster than light in a vacuum. However, light moves slower in a medium, and it is possible to travel faster than light in the medium as long as you are travelling slower than light in a vacuum.) When a charged particle travels faster than light in a medium, it slows down and emits light in the direction of motion. This is called Cerenkov radiation, and it is the source of the pretty blue glow surrounding water immersed nuclear reactors. Charged particles travelling slower than light can also slow down by collisions and emission of radiation in the direction of motion, called Bremsstrahlung radiation. Both Cerenkov radiation and Bremsstrahlung radiation resulting from cosmic rays can, and do, contribute to airglow. In fact, there is a whole new branch of astronomy that studies cosmic rays through their Cerenkov radiation. Â
Much of the airglow that I have mentioned is visual light. But, the air itself gives off light simply by not being at absolute zero. Any object that is not at absolute zero gives off light. The hotter it is, the more light that it gives off and the more shorter wavelength light that it emits. This is called blackbody radiation, or sometimes thermal radiation. Since the air is not at absolute zero, it emits light. Most of the light given off this way in our atmosphere is infrared, which limits how effective an infrared telescope can ever be without getting above the Earth’s atmosphere. Â
So, while we might think that sunset means totally dark skies, that just isn’t the case.  To really have totally dark skies, you have to have no atmosphere. That is the beauty of space-based telescopes.
-Astroprof
(Image courtesy of NASA)
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A Ler…-- Rastos de Luz on November 18, 2006 at 6:14 am: 1
[…] “Airglow“, no Astroprof’s Page; […]
Doug Zubenel on December 4, 2006 at 3:37 pm: 2
Hello Astroprof,
I enjoyed the your airglow article. I fyou look in the July 2006 issue of Sky and Telescope, you will see an image I made of a banded (my word) airglow display at the 2005 Nebraska Star Party.
S&T’s Steve O’meara will have an article about airglow in the Feb. 2007 issue of S&T.
Later,
Doug Zubenel
Astroprof on December 4, 2006 at 4:30 pm: 3
Thanks for you comment. I’ll be sure to be watching for the upcoming S&T article.