Timelapse footage of the Aurora Borealis, or northern lights, in a moon-lit sky. Auroral displays are caused by interactions between energetic charged particles from the Sun, and the Earth's atmosphere. Moving at 400-500 kilometres a second, the charged particles of the solar wind are drawn by Earth's magnetic field to the poles, where they collide with gas atoms and molecules, causing them to emit light. Green light is from oxygen atoms, purple light from nitrogen atoms. Filmed in Minnesota, USA, in winter.