what causes an aurora?

These colourful displays are called the northern lights (or aurora borealis) in the northern hemisphere, and the southern lights (or aurora australis) in the southern hemisphere.

By making measurements of the solar wind from observatories in the Arctic, I can predict when we'll see the northern lights. If they were really just pretty lights in the sky this would be interesting but not very important. However, aurorae can be the warning lights for some real problems on and around the Earth.