How the Aurora Works

The aurora is the visible end of a supply chain that starts at the Sun. Solar wind — a continuous flow of charged particles — piles into Earth's magnetic field, stretches it into a long tail on the night side, and occasionally snaps it into a new configuration. That snap slings particles down the field lines toward the poles.

The colors are chemistry: excited oxygen glows green at low altitude and blood-red higher up; nitrogen edges the curtains in purple. A strong storm pushes the oval of visibility toward the equator, which is why a big coronal mass ejection can put the northern lights over places that almost never see them.

Photo: NASA -- US government agency, via Wikimedia Commons (Public domain).