Airglow

The night sky has an overall background luminosity. We are aware of the localized sources of light in a moonless night sky (the stars and planets, the zodiacal light, and gegenschein), but in addition to the astronomical sources there is an overall uniform luminosity originating from the Earth's own atmosphere. We are not normally aware of this airglow because it is so uniform. It is the combination of astronomical and airglow sources that allows us to see the silhouette of an object held against the "dark" sky on a clear moonless night.

Airglow refers to the luminosity (glowing) of the atmosphere
The brightest region of airglow is about 10 mile (10 to 20 km) thick zone at an altitude of about 60 miles (100 km). One contributor to airglow is the sodium layer.

Airglow viewed from space. The orangish arc extending to the left from the aurora is airglow viewed edgewise on the limb of the Earth.

Airglow: The Movie Video and commentary from the Int'l Space Station by astronaut Don Pettit
Also take a look at NASA's Space Station Science Picture of the Day for April 3, 2003.
Airglow is commonly divide into:
- Dayglow (when entire atmosphere is illuminated by the Sun) is the brightest airglow due to the importance of RESONANT and FLUORESCENT processes (see below) but it is overwhelmed by direct and scattered sunlight
- Twilightglow (when only the upper atmosphere is illuminated) is the most readily observable airglow from the ground since the observer is in darkness (and Rayleigh scattering of sunlight by the dense lower atmosphere is absent) while the airglow region of upper atmosphere is still illuminated
- Nightglow (when entire atmosphere is in darkness) is not as bright as dayglow since CHEMILUMINESCENCE (see below) is the dominant process; however contributes more light than starlight to the total luminosity of the night sky
Airglow vs aurora
- spatial extent: global vs. high latitude
- luminosity: relatively uniform vs. highly structured
- energy source: solar radiation vs. solar wind
Airglow is due to emission from excited states formed by processes resulting (directly or indirectly) from solar radiation. These processes include:
- RESONANCE: emitted light is at same color as that absorbed resulting from excitation by the absorption of solar radiation
- FLUORESCENCE: emitted light is at lower frenquency, i.e. a different color, resulting from excitation by the absorption of solar radiation
- PHOTOIONIZATION: emitted light is from the excited states of ionized fragments caused by solar radiation
- PHOTODISSOCIATION: emitted light is from the excited states of neutral fragments caused by solar radiation
- INELASTIC COLLISIONS: emitted light results from excitation caused by the impact of high energy ("hot") electrons that are produced in photoionization [NOTE: This is the same process that causes the luminous aurora but in that case the "hot" electrons come from the magnetosphere through interactions with the solar wind.]
- CHEMILUMINESCENCE: emission results from chemical reactions mainly between oxygen and nitrogen atoms and molecules and hydroxyl molecules at a height between 100 and 300 kilometers. Solar radiation energy breaks molecules apart during the day, and it is their recombination, which is accompanied by the emission of light, that generates the nightglow.
- EXCITATION by COSMIC RAYS (high energy radiation and particles from outside the solar system) make a small contribution to airglow as well.
During magnetic storms the airglow at mid-latitudes is modified by heating effects, producing stable auroral red (SAR) arcs (as seen on the right edge of the image). These are broad diffuse arcs of light that align along geomagnetic parallels between about 40-55 degrees and located at a height of 300-400 kilometers. They are quite bright in red oxygen emission but are subvisual because the human eye's sensitivity at that wavelength is only about 5% of its peak sensitivity in the yellow region of the spectrum.
Brief Historical Outline

1868	Anders Angstrom discovers green line is present in the night sky even when no aurorae are present
1920's	Robert John Strutt (4th Baron Rayleigh) begins investigations [Note: he is referred to as the "airglow Rayleigh"; his father John William Strutt, 3rd Baron Rayleigh, is the "scattering Rayleigh"]
1923	John McLennon & G.M. Shrum identify green line to be due to atomic oxygen
1929	Vesto Melvin Slipher discovers sodium layer (a contribution to airglow)
1931	Sydney Chapman suggests airglow is result of chemical recombination
1939	Chapman suggests reaction cycle to sustain sodium nightglow
1950	term "airglow" coined after other atmospheric emissions are identified
1956	SAR arc discovered by Barbier over France