(Also called light-of-the-night-sky, night-sky light, night-sky luminescence, permanent aurora.) The quasi-steady radiant emission from the upper atmosphere over middle and low latitudes, to be distinguished from the sporadic emission of auroras that occur over high latitudes. Airglow is a photochemical luminescence (or chemiluminescence) arising from chemical reactions in the upper atmosphere. Many of these reactions leave molecules and atoms in excited states from which they can radiate at certain well-defined wavelengths. Emissions from molecular oxygen O2, atomic oxygen O, sodium Na, and the hydroxyl radical OH are especially prominent, and measurements of airglow intensity by spectrometric techniques have provided a great deal of information about upper-atmospheric dynamics and chemistry.

Faint, visible and infrared emission in Earth's upper atmosphere caused by solar radiation.

A faint glow of the night sky, present even in "perfectly dark" areas, caused by the recombination of particles in the upper atmosphere.

The quasi-steady radiant emission from the upper atmosphere as distinguished from the sporadic emission of the auroras. See discussion p. 66-67.

The visible light that appears at night in the upper atmosphere.

The faint luminosity of the night sky, due to the Earth's upper atmosphere.

The airglow is the very weak emission of 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.