What are the northern lights?

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What are the northern lights?
Aurora borealis
Aurora Borealis over Iceland, by Moyan Brenn

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 charged gas, called a coronal mass ejection. After two or three days, these charged particles reach Earth, where the planet’s magnetic field deflects them toward the North and South Poles.These charged particles can excite a molecule or atom if they collide, and when these molecules or atoms return to their normal energy states, light is emitted.

Auroras form between 60 and 250 miles above the Earth’s surface when charged solar particles collide with two primary constituents of our atmosphere: nitrogen and oxygen. Nitrogen molecules emit pinkish or magenta light, while oxygen atoms emit greenish light. A majority of the collisions occur near the poles, so the northern lights usually appear at the higher latitudes of Canada and Alaska. When the sun emits a large number of particles, usually during a solar flare, the lights from the collisions can be seen further south.

Our sun’s activity changes. It was very quiet from 2008 to 2010 , but the sun has become more active, so we may have an opportunity to see more of these great natural light shows. Here is a photo of the northern lights taken from Middleton, Wis. by J. Zhou on March 10, 2011.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.