Green and purple curtains of the aurora borealis (Northern Lights) appeared in the skies over Alaska in this pair of photos from March 2001. Energetic particles (mostly electrons and protons) in Earth's magnetosphere collide with atoms and molecules in the upper atmosphere. The collisions add energy to the atoms and molecules, which in turn emit the energy as light. Oxygen atoms give off the green light seen in these two photos, while nitrogen molecules emit the purple hues.
Images courtesy of Jan Curtis.