Airglow or nocturnal luminescence is a weak light emission from the Earth’s atmosphere; as a result, the night sky is never completely dark. It was first noticed in 1868 by Anders Jonas Ångström and it is caused by a set of processes in the upper layers of the atmosphere, such as the recombination of ions that have been photoionized by solar radiation during the day, the luminescence resulting from collisions between the constituents of the atmosphere and incident cosmic rays and the chemiluminescence associated with the reactions of oxygen and nitrogen with the hydroxide ion at heights of a few hundred kilometers. The phenomenon is not visible during the day because it is concealed by sunlight.
Airglow reduces the sensitivity limit of ground-based telescopes of even the largest astronomical observatories. Partially for this reason, space telescopes can observe objects dimmer in the visible lengths than ground-based telescopes.
Nighttime airglow can be bright enough to be noticed by an observer and is generally blue in color. Although the emissions are fairly uniform throughout the atmosphere, to an observer on the surface it appears brightest at about 10° above the horizon because the lower he or she looks, the greater the atmospheric thickness he or she traverses with his or her gaze. However, the apparent brightness of airglows is reduced by the light absorption of the atmosphere.
One mechanism that produces aiglow is the combination of a nitrogen atom with an oxygen atom to form a nitrogen monoxide (NO) molecule.
During the process, the emission of a photon occurs. The photon can be emitted at different wavelengths characteristic of the molecule. Free atoms are available in the upper atmosphere where nitrogen (N2) and oxygen (O2) molecules are dissociated by solar radiation. Other chemical species that can contribute to aiglows include: hydroxide ion (OH–), hypoiodite ion (OI–), and sodium iodide (NaI).
Sky brightness is typically reported in units of astronomical magnitude per square arcsecond of sky (m/”²).
Similar emissions have been identified on the other planets of the Solar System equipped with an atmosphere and on some main moons, for which the detection of these emissions has allowed to establish the existence of a thin atmosphere.