This is a repost with updates from January 2004. But with some great new resources.
I confess. The Northern (and Southern) lights fascinate me and one of my dream trips is to return to Northern Europe when the northern lights are blazing. The Northern lights is the result of our atmosphere shielding against solar particles which would otherwise make our planet uninhabitable. During large explosions and flares on the sun, huge quantities of solar particles are thrown out of the sun and into deep space. These plasma clouds travel through space with speeds varying from 300 to 1000 kilometers per second. But even with such speeds (over a million kilometer per hour), it takes these plasma clouds two to three days to reach our planet. When they are closing in on Earth, they are captured by Earth's magnetic field (the magnetosphere) and guided towards Earth's two magnetic poles; the geomagnetic south pole and the geomagnetic north pole.
On their way down towards the geomagnetic poles, the solar particles are stopped by Earth's atmosphere, which acts as an effective shield against these deadly particles.
When the solar particles are stopped by the atmosphere, they collide with the atmospheric gases present, and the collision energy between the solar particle and the gas molecule is emitted as a photon - a light particle. And when you have many such collisions, you have an aurora - lights that may seem to move across the sky.
In order for an observer to actually see the aurora with the naked eye, about a 100 million photons are required.
One of the best sites for more information is in Northern Norway. There is also a superb site covering the Aurora Australis or Southern Lights. When I was in Australia, one of the best shows in the evening was simply to walk outside. Once you get out of Melbourne or Sydney, where there is a lot of light pollution, the night sky just opens up. If there is any storm activity, the night sky is just fantastic. Makes you feel very small though.
A few years ago I remember seeing a documentary on the now defunct BBC HD network. It has now made its way onto YouTube. Best one I have every seen on the Northern Lights. Beautifully done.