airglow, faint diffuse illumination of the sky originating in the upper atmosphere. Although it occurs at all times of day throughout the upper atmosphere, it is most typically visible to an observer on earth on dark nights, above the horizon. The energy in the form of visible light is derived from the sun's ultraviolet light, which ionizes atoms and dissociates molecules at heights between 40 and 200 mi (64–322 km) above the earth's surface. When the fragments collide and recombine, some atoms and molecules are left with excess energy, which they release as light at characteristic wavelengths. Most prominent in the visible spectrum are the red and green light of oxygen and the yellow light of sodium. In southern and northern polar regions the airglow is often masked by the aurora (see aurora borealis). Airglow hampers optical telescopic observations on earth by reducing the apparent contrast between stars and space.

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