Airglows of many colours

Left: Yuri Beletsky imaged this at Carnegie Las Campanas Observatory, Chile.

Against a backdrop of the galaxy, two Magellanic Clouds and Orion there is airglow of two, or possibly three, colours.

Deep red banded airglow spans the sky. It is likely emission from vibrationally excited OH radicals in a layer ~86km high. The bands are caused by gravity waves propagating upwards from the lower atmosphere. They modulate the local pressure, temperature and specie concentrations.

The green airglow of oxygen atoms (1S ->1D) 90-100 km high patches the closer horizon sky.

The red/orange patch at top right could be yet more oxygen airglow, this time from atoms 150-300km high where the atmosphere is so sparse and collisions so infrequent that the atoms have time to radiate 'forbidden' light (1D ->3P) before losing their electronic excitation in impacts with other atoms and molecules.

Faint blue airglow is sometimes seen.

mages ©Yuri Beletsky, shown with permission.

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A different airglow palette. Some green airglow can be seen lighting the sky but the predominant colour is a red/orange, probably high level (150-300km) radiation from excited oxygen atoms.

Airglow is the light of electronically and/or vibration-rotationally excited atoms and molecules high in Earth's atmosphere..

Aurorae are at similar heights and they too are the light of excited atoms. There is a difference, auroral excitation is by collisions with energetic particles whereas daytime short wavelength solar radiation produces the excitation.

The sun’s extreme ultraviolet light excites oxygen and nitrogen atoms and molecules in the thermosphere*. The energetic products collide and interact with other atmospheric components, including hydroxyl radicals (OH), to eventually produce light emission by chemiluminescence*and the decay of excited atoms and molecules.

Airglow is weak, always weak. Take away manmade light pollution, take away dust and haze, take away moonlight. The final barrier to a sky lit only by the stars is.. ..airglow.