The aurora happens because super-fast charged particles sneak into the Earth's atmosphere and collide with ambient atoms and molecules, which release light. The beautiful variety of colors of the aurora are due to the different types of atoms and molecules in the atmosphere. For example, nitrogen, hydrogen, and oxygen molecules tend to emit red light; oxygen also emits green light, and nitrogen ions emit violet/purple light. These nitrogen ions are only found in the highest parts of the atmosphere, at about 1000 km altitude, so you often don't see them. If you get the opportunity to see the aurora from right angle, you might see the top of the aurora glowing purple.
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