Airglow, also known as night glow, is a faint, persistent emission of light from a planetary atmosphere. Unlike starlight or moonlight, which reflect and scatter light from celestial sources, airglow is produced by chemical reactions within the Earth's upper atmosphere. This optical phenomenon ensures that the night sky never appears completely dark, even when viewed from the far side of the Earth, away from direct starlight.

The phenomenon was first identified in 1868 by Swedish scientist Anders Ångström. Since then, airglow has been studied in both laboratory settings and through astronomical observations, revealing various processes that produce this light emission.

Airglow results from chemical reactions occurring in the Earth's thermosphere and exosphere, primarily driven by solar ultraviolet radiation. This includes:

• Green Airglow: Emission at a wavelength of 5577 Angstroms, caused by the recombination of oxygen atoms.
• Red Airglow: Emission at 6300 Angstroms, also from oxygen atoms, but at a different energy state.
• Yellow Airglow: Emission at 5893 Angstroms, resulting from sodium atoms.

Airglow has a daytime counterpart known as dayglow or twilight airglow. During daylight, solar radiation energizes the upper atmosphere, causing it to emit light. This phenomenon is often overshadowed by the brightness of the sun but can be significant at twilight.

Night airglow, on the other hand, persists after sunset due to delayed reactions from absorbed solar energy. It is noticeable as a soft, diffuse light that can sometimes exceed the brightness of distant stars. This phenomenon is particularly prominent in high-latitude regions where it can provide substantial nocturnal illumination.

The brightness and intensity of airglow can vary based on factors such as solar activity and atmospheric conditions. Observations from extremely dark locations, away from artificial light pollution, reveal more vivid airglow effects. For example, images of airglow taken from remote locations like the Atacama Desert show striking green hues in the night sky, which are often mistaken for auroras.

Despite its significant contribution to nighttime illumination, airglow has not been extensively studied compared to other atmospheric phenomena like auroras. Researchers continue to investigate its exact causes and implications for atmospheric science. Airglow's subtle yet pervasive presence serves as an important reminder of the dynamic and complex processes occurring in our atmosphere.

• SpaceWeather.com (March 21, 2016): Observations of intense green airglow over the Atacama Desert highlighted its resemblance to auroras but clarified that its causes are different. Photographs revealed airglow's significant impact on the night sky, overshadowing other celestial objects like the Magellanic Clouds.
• Far from Black: Night airglow provides a soft illumination that can be brighter than starlight in certain conditions, especially in snowy landscapes, making it a notable source of natural light at night.