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The airglow is the very weak emission of visible light by the earth's atmosphere, which means that the night sky is never completely dark. It was first noticed in 1868 by Anders Ångström. It is caused by various processes in the upper atmosphere, such as the recombination of ions which were photoionised by the sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl ions at heights of a few hundred kilometres.
The airglow may be bright enough to be noticed by an observer. Although airglow emission is fairly uniform across the atmosphere, to an observer on the ground it appears brightest at about 10 degrees above the horizon, because the lower one looks the greater the depth of atmosphere one is looking through. Very low down, however, atmospheric extinction reduces the apparent brightness of the airglow.
See also
Polar aurora
Optical phenomena
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