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The Milky Way spans the sky accompanied by the dwarf galaxies of the Magellanic Clouds. Suffusing all is a much closer light, ‘airlight’ from our own upper atmosphere.
The green and red banded glow is produced by atoms and molecules whose exitation derives ultimately from daytime far ultra violet radiation.
The green light is from excited oxygen atoms radiating to a lower excited state (O 1S
to 1D) at an altitude of 90 – 100km (56 - 62 miles).
The narrow altitude range is the result of competition between the slow radiative decay (~1s) and de-excitation by collisions coupled with the variation of atomic oxygen concentration with height. The red glow is possibly from OH radicals. They also emit in a narrow altitude range.
The upper atmosphere is perturbed by gravity waves emanating from disturbances at lower levels. The waves modulate the atmospheric density and temperature with subsequent effects on the balance of the airglow processes that then result in a banded glow.
To see airglow demands a long dark adaption and a moonless sky free of light pollution.
Luis Argerich's exposure was 30s at ISO 3200, 14mm lens at f/2.8 Canon EOS 5D.
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