NASA ISS: Aurora Borealis from the ISS

Earlier this week, NASA astronaut Clayton Anderson tweeted a 14 second time-lapse film of the Aurora Borealis taken from the International Space Station.

The short clip called to mind a more extensive view of the Northern Lights shot by Don Pettit, also working in the ISS, back in 2008 and it raised the basic question: What causes the Aurora Borealis anyway?