Why was part of the sky green on Tuesday night?


Photo credit: Garrett Frankson


If you were out on Tuesday night sometime between 8 and 9 p.m., you might have seen the sky glow green. It wasn’t a St. Patrick’s Day mirage; it was the northern lights, also called aurora borealis, which appear as a diffuse glow or as overlapping curtains of greenish-white and sometimes red light in the sky.

Auroras are triggered when the sun ejects a cloud of gas, called a coronal mass ejection. It takes about two or three days for the charged particles in this gas to reach Earth.

Earth’s magnetic field deflects these particles toward the North and South Poles. When these charged particles collide with a molecule or atom they can excite the molecule, increasing their energy state. When these molecules or atoms shift back down to their normal energy states they emit 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 emits pinkish or magenta light, while oxygen emits greenish light.

Most of the collisions occur near the poles, so the northern lights are usually seen at the higher latitudes of Canada and Alaska.

The northern lights were particularly visible from southern Wisconsin. It is not very common for the northern lights to reach this far south.

Category: Meteorology, Phenomena

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