Environmental Graffiti has a roundup of amazing light phenomena photos, with nice pithy explanations of the science. Via Mr.Reid

The collision of electronically charged particles in the earth’s upper atmosphere often creates magnificent light displays over the polar regions. The colour depends on the elemental content of the particles – most auroras appear green or red due to oxygen, however nitrogen sometimes creates a deep blue or violet appearance. This particular display is the famous Aurora Borealis or Northern Lights, named after the Roman goddess of dawn, Aurora, and the ancient Greek for the northern wind, Boreas.