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AN AURORA FORMS IN THE SKIES OF FAIRBANKS, ALASKA

An aurora forms a green arc that stretches across in the early evening sky in Fairbanks, Alaska on February 10, 2007. Auroras are formed from electrons in the atmosphere that get accelerated in regions between about 5000 and 8000 kilometers in altitude, and rush down to the Earth's ionosphere, a region of the upper atmosphere. The electrons then crash into ionospheric atoms and molecules, transferring their energy to cause a green glow. The length of an aurora arc can be as large as several thousands kilometers, but its width can be as small as 100 meters. (UPI Photo/Jan Curtis/ESA)

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