If you look closely at the night sky you will see that, at the horizon line, you will see a faint green glow. This is called airglow and happens allover the world. The lower parts of it are green, but in the upper atmosphere the color is actually red.
What causes this airglow?
The excited oxygen atoms do this. During the day the UV rays hits the O2 molecules and, due to the high energies of UV rays, the molecule are split up in oxygen atoms that also engulf some of the UV energy. We call these oxygen atoms that have some extra UV energy in them excited atoms.
After a while the oxygen atoms combine into a molecule back again releasing a faint blue light. If you go between 95 – 100 km int he atmosphere you will encounter lone oxygen atoms that release the energy in the form of visible green light.
If you go above 100 km you will encounter excited oxygen atoms that will release a red light. The atoms become stable again by moving the electrons from higher energy states, where they wound up by eating some of the UV energy, to their stable or ground energy states. By moving the electrons back to the ground states a photon is emitted.