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faint luminescence of the Earth's upper atmosphere that is caused by air molecules' and atoms' selective absorption of solar ultraviolet and X-radiation. Most of the airglow emanates from the region about 70 to 300 km (42 to 180 miles) above the surface of the Earth. Unlike the aurora, airglow does not exhibit structures such as arcs and is emitted from the entire sky at all latitudes at all times. The nocturnal phenomena is called nightglow. Dayglow and twilight glow are analogous terms