What causes the Aurora Borealis?
Originally posted by LURCH
It's explained pretty well Here . Basically, the Sun emits particles (called the Solar Wind) which the Earth's magnetic field deflects away from the surface. But at the poles, that magnetic field "curves downward" and comes into direct contact with the surface. Charged particles from the Sun can ride down these nearly vertical lines of magnetic force. As these particles reach the ionosphere, the charges in that layer of the atmosphere interct with the charge in the Solar wind, giving off visible light, radio static, etc.