The Telegraph has published a gallery with pictures of the Northern Lights taken in the last few years from the ISS and several space shuttles. They are extremely beautiful. Follow the link to view the gallery.
The Northern Lights, or Auroras are caused by the emissions of protons above 80km (50 miles) in the atmosphere, "from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state.
They are ionized or excited by the collision of solar wind particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule.