|Aurora to the left, city lights to the right|
The best display was before sunrise while I was still asleep. Fortunately, it persisted throughout the day, so I went out last night to have a look.
The Aurora Borealis as it is known (aka the Northern Lights) is caused by a curious interaction between the solar wind and the Earth's magnetic field. There is always an Aurora Borealis and Australis going on, but normally it is restricted to the Arctic and Antarctic circles respectively.
On occasion, a CME (Coronal Mass Ejection) is directed at Earth. This stream of charged particles plays havoc with our magnetic field. As the particles collide with each other over Earth's atmosphere, photons are released. This is what creates the spectacular light shows that we see. The more particles there are hitting the Earth, the stronger the light show, and, usually the farther south it can be seen.
Last night, the aurora was seen as far south as North Carolina. Pretty impressive.
Below are some more photos:
Or view them on Flickr: