Circles in the Sky
by Alan Dyer
When electrons and protons beam down from the magnetic fields around our planet, they form a unique pattern in the sky. Seen from space, our atmosphere lights up in two glowing rings, each about 4,000 kilometers wide. One ring is centered on the north magnetic pole in the Canadian Arctic, while the other is centered on the south magnetic pole in Antarctica. These rings are called “auroral ovals”—a bit misleading, as the rings are usually round.
They form because Earth behaves as if it has a giant bar magnet inside the planet. The “force-field” lines from Earth's magnetic interior extend out from the tips of the core magnet like a widening cone. When solar particles rain from space they follow the magnetic field lines down to Earth, trying to converge on the tip of the bar magnet deep inside the planet. The particles hit the atmosphere first, creating twin glowing rings of light. At the bottom of the world, the southern oval creates aurora australis, the southern lights, a mirror image of the more familiar northern lights.
In North America, the northern auroral oval usually shimmers over Alaska and Canada's northern territories, where people see northern lights almost every night. During a powerful solar storm, the auroral oval expands and heads south, bringing glowing lights to the skies of the southern United States and, on rare occasions, even Mexico.
- Why do auroral ovals normally occur at the northern and southern poles and not over other parts of Earth?
- What might cause an auroral oval to appear in a place where it normally does not?
- What could be happening if there were no auroral ovals over any part of Earth?