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July 14, 2011
City lights and two separate atmospheric optical phenomena were captured in this 35mm camera's time exposure photograph from the Space Shuttle Endeavour. The thin greenish band stretching along the Earth's horizon is airglow; light emitted by the atmosphere from a layer about 30 kilometers thick and about 100 kilometers in altitude. The predominant emission in airglow is the green 5577 Angstrom wavelength light from atomic oxygen atoms. Airglow is always and everywhere present in the atmosphere; it results from the recombination of molecules that have been broken apart by solar radiation during the day. But airglow is so faint that it can only be seen at night by looking "edge on" at the emission layer, such as the view astronauts and cosmonauts have in orbit. The second phenomenon in the photo is the green color of the aurora borealis, or "northern lights". Bright patches of aurora are superimposed on the fainter airglow, and a band of aurora is also clearly visible in the far right side of the scene. Aurora occur from about 100 km to 300 km altitude only in the auroral zones at polar latitudes. The green color is also caused by the emission of 5577 Angstrom wavelength light from oxygen atoms that have been raised to a higher energy level (excited) by collisions with energetic electrons pouring down from the Earth's magnetosphere. The light is emitted when the atoms return to their original unexcited state. Astronauts often comment that their photography does not do justice to just how spectacular the view of the aurora is from space. The unidentified city lights are smeared because of the time exposure.

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