Auroras are caused by the collision of charged particles in the magnetosphere with atoms in the Earth’s upper atmosphere.

These charged particles are energized to levels between 1,000 and 15,000 electron-volts and as they collide with atoms of gases in the atmosphere the atoms become energized, then emit their energy as light.

For those of us that will likely never be fortunate enough to see the Aurora Borealis in person, here is a time lapse video of one of the most extraordinary sites in nature. Wikipedia also has a fantastic entry for Auroras.
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