(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.