The northern lights, also called aurora borealis, are an evening light show seen as a diffuse glow or as overlapping curtains of greenish-white and sometimes red light. Auroras are triggered when the surface of the sun ejects a cloud of gas, called a coronal mass ejection. It takes about 2 to 3 days for the charged particles in this gas to reach Earth. Earth’s magnetic field deflects these particles towards the North and South Poles. When these charged particles collide with a molecule or atom they can excite them. When these molecules or atoms shift back down to their normal energy states they light.
Auroras form between 60 and 250 miles above the Earth’s surface when these charged solar particles collide with two abundant constituents of our atmosphere: nitrogen and oxygen. Nitrogen molecules emit pinkish or magenta light, while oxygen atoms emit greenish light. A majority of the required collisions occur near the poles, so the northern lights are usually seen at the higher latitudes of Canada and Alaska. When a large number of particles are emitted by the sun, which usually happens after a solar flare, the lights from the collisions can be seen at our latitude.
Our sun goes through active and quiet periods. The time between 2008 and 2010 was a very quite time, but now the sun is becoming more active so we may have an opportunity to see more of these great natural light shows. Here is a recent photo of the northern lights taken on March 10, 2011 from Middleton by J. Zhou http://tinyurl.com/67beo59.