Tag: hydroxyl radicals

The softly glowing night sky

By Phil Plait | June 11, 2012 6:57 am

There’s a lot more going on over your head than you know.

For example, the atmosphere of the Earth thins out gradually the higher you go, and when you get to about 100 kilometers (60 miles) up, different physical processes become important. One of them is called chemiluminescence — light produced by chemical processes. This can make the upper atmosphere glow in different colors. It’s faint, and best seen from space… where we conveniently keep several astronauts. Neuroscientist and amateur video maker Alex Rivest has collected pictures of this airglow taken by astronauts and made this eerie and beautiful time lapse video:

Alex took the original astronaut pictures and enhanced them somewhat to bring out the faint airglow. You can see it in lots of pictures taken from the space station, and I’ve commented on it many times. One thing I’ve been meaning to do, though, is find out what the physical process is that’s causing the air to glow, and why it creates different colors — you can clearly see green, yellow, and red glow in many of the pictures!

Alex comes to the rescue on that as well. On his blog, he discusses how he made the video and why the air glows (based on a somewhat terser explanation at the Atmospheric Optics website).

The way this works is simple in general, though complicated in detail — much like everything else in the Universe! Basically, during the day, in the upper atmosphere ultraviolet light from the Sun pumps energy into oxygen molecules (called O2; two oxygen atoms bound together — this is the stuff we breathe). This energy splits the molecules apart into individual atoms, and these atoms have a little bit of extra energy — we say these atoms are in an excited state. Like a jittery person who’s had too much coffee, they want to give off this energy. They can do this in a couple of ways: they can emit light, or they can bump into other atoms and molecules and react chemically with them.

If you have an excited oxygen atom sitting in space all by its lonesome, it can either dump that energy by emitting green light or red light. Usually, it’ll emit green light in less than a second after becoming excited, and it’ll emit red light on much longer timescales, like minutes. This is important, so bear with me.

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