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Glowing of the atmosphere

On Earth, glowing oxygen is produced during polar auroras when energetic electrons from the Sun hit the upper atmosphere. This oxygen-driven emission of light gives polar auroras their beautiful and characteristic green hue.

The aurora, however, is just one way in which planetary atmospheres light up. The atmospheres of planets including Earth and Mars glow constantly during both day and night as sunlight interacts with atoms and molecules within the atmosphere, a phenomenon commonly called “airglow”.

Day and night glow are caused by slightly different mechanisms.

Night: returning to original state

Nightglow results from the recombination of molecules that have been broken apart by solar radiation during the day. The light is emitted when the excited atoms or molecules return to their original unexcited state during the night. One of the brightest emissions on Earth stems from night glow, more particularly from excited oxygen atoms radiating light at a particular wavelength of light (557.7 nanometer), a green radiation emitted in the spectral region where our eyes are most sensitive.

The green layer of nightglow is quite faint so that it can only be seen at night by looking "edge on" at the emission layer, such as astronauts can view it in orbit. This optical phenomenon was captured with cameras on board the Space Shuttle and the International Space Station (Figure 1).

Daytime: absorbing sunlight

Dayglow arises when the Sun’s radiation directly interacts with the daytime atmosphere. Atoms and molecules (such as nitrogen and oxygen) in the atmosphere absorb some of the sunlight, which excites them temporarily until they release this additional energy as light, either at the same or lower frequency (colour) than the absorbed light.

This emission is much weaker than the light scattered from the Sun, so we cannot see it with the naked eye.

Nightly airglow or "nightglow" of Earth's atmosphere is clearly visible in this picture, taken at Mugecuo lake in eastern Tibet. Credit: Petr Horalek - "Reaching the Mugecuo lake" -
Figure 1: Image of the night side green airglow layer surrounding the Earth near 90 km, observed from the International Space Station.