Photograph of a beautiful curved band of aurora over the snowy Swedish Lapland. The phenomenon of  Northern Lights (also called Aurora Borealis) occurs when large numbers of electrons stream in towards the Earth along its magnetic field and collide with air particles. The air then lights up rather like what happens in a fluorescent light tube. The resulting colors of the Northern Lights reflect the gases that we find up there. The charged particles originate from the sun, and the weather conditions on the sun decide whether or not we will see the aurora. Photography by  Antony Spencer