Aurora - light emission by excited atoms

(a) - a high-energy particle from the magnetosphere collides with an air molecule in the upper atmosphere

(b) the air molecule is excited - has more energy

(c) the air molecule then gives off this excess energy in the form of light in order to get back to the original energy level

Here at LSC, we have seen very nice aurora displays on clear fall and winter nights - check out the images at http://www.vtphoto.com