Airglow

Airglow is a faint emission of light from a planetary atmosphere that keeps the night sky from ever being fully dark, even once starlight and any diffused sunlight from the far side of the planet are removed. It comes from self-illuminated gases in the atmosphere and is a separate phenomenon from the aurora, which instead involves the magnetic field of Earth and incoming solar particles. It has two forms: dayglow, when sunlight breaks apart atmospheric molecules during daylight hours though it is too faint to see against the bright sky, and nightglow, when those split molecules recombine at night; airglow is also produced by recombination of atoms photoionized by the sun, by cosmic rays striking the upper atmosphere, and by chemiluminescent reactions between oxygen, nitrogen and hydroxyl radicals hundreds of kilometres up, giving it a generally bluish appearance that is brightest about 10 degrees above the horizon.