How are the northern lights produced?

Although I've never seen it myself, I hear the northern lights are a sight to be seen! I know they're related to the Earth's magnetic field but I don't know much more about them. What is the physical phenomenon that creates the northern lights?

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The ionized particles from mainly solar wind are caught and trapped by Earth magnetic field, which behaves like a magnetic bottle. (The region in which ions are trapped is called Van Allen radiation belts.) This trap is weaker in the polar regions, and there the ions are mainly released into the denser parts of atmosphere. There they collide with air particles (mostly N$_2$ and O$_2$) causing their fluorescence seen as the northern lights.

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A more detailed explanation can be found in Wikipedia: en.wikipedia.org/wiki/Polar_aurora –  mbq Nov 3 '10 at 16:56
Actually, the field is stronger at the poles. The magnetic field directs charged particles toward the poles, and there they collide with the atmosphere. –  grizzly adam Nov 4 '10 at 3:42
@grizzly That's why I wrote that the trap is weaker. –  mbq Nov 4 '10 at 8:28
Perhaps it would be clearer if you said that the magnetic field lines "lead to" or "gather at" the poles. –  Mark C Nov 4 '10 at 21:26