Sky Brightness - Airglow

Airglow

When physicist Anders Ångström examined the spectrum of the aurora borealis he discovered that even on nights when the aurora was absent its characteristic green line was still present. It was not until the 1920s that scientists were beginning to identify and understand the emission lines in aurorae and of the sky itself and what was causing them. The green line Angstrom observed is in fact an emission line with a wavelength of 557.7 nm caused by the recombination of oxygen in the upper atmosphere.

Airglow is the collective name of the various processes that occur in the upper atmosphere that result in the emission of photons with the driving force being primarily UV-radiation from the sun. Several emission lines are dominant, a green line from oxygen at 557.7 nm, a yellow doublet from sodium at 589.0 and 589.6 nm, and a red lines from oxygen at 630.0 and 636.4 nm.

The sodium emissions come from a thin sodium layer approximately 10 km thick at an altitude of 90 - 100 km, above the mesopause and in the D-layer of the ionosphere. The red oxygen lines originate at altitudes of about 300 km, in the F-layer. The green oxygen emissions are more spatially distributed. How sodium gets to mesospheric heights is not yet well understood, but it is believed to be a combination of upward transport of sea salt and meteoritic dust.

In daytime sodium and red oxygen emissions are dominant, and are roughly 1000 times more luminous than nighttime emissions because in daytime the upper atmosphere is fully exposed to solar UV radiation. The effect is however not noticeable to the human eye since it totally fades in the glare of directly scattered sunlight.