Airglow over the VLT (Very Large Telescope) platform.
Airglow, a faint emission of light by a planetary atmosphere, is caused by various processes in the upper atmosphere, such as the recombination of atoms which were photoionized 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. It is not noticeable during the daytime because of the scattered light from the sun. The airglow at night may be bright enough to be noticed by an observer and is generally bluish in colour.
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