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An airglow is diffuse radiation, extending from the near infrared to the far ultraviolet, that is continuously emitted by a planetary atmosphere; also known as nightglow. It is caused by the collision of charged particles and X-rays from space, mainly from the Sun, with atoms and molecules high in the atmosphere.

Earth's airglow varies with the time of night or day, latitude, and season, goes from a minimum at the zenith to a maximum about 10° above the horizon, and arises mainly from discrete atomic and molecular transitions that give rise to a mostly emission-line and emission-band spectrum. Emission from oxygen at 5577 Å (green) predominates at night, while yellow sodium and red oxygen emission is prominent at twilight. Daytime airglow, though drowned by sunlight, is actually 1,000 times as intense as at night.

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