They're created by ionized sunlight interacting with the planet's magnetic/gravitational field. And wouldn't ya know it, other planets have gravity and poles too.
They're circular because of the WAY they interact. I couldn't explain it, mostly because I'm don't quite know. But I know it only happens in a circle around the poles of a planet.
They're circular because what other shapes would they have? Heptagons? That would take some explaining to do. The geometry if the effect follows the geometry of the causes. The magnetosheet discharges itself into the poles, so we've got two 'points' where stuff happens. Widen points and you've got circles. Circles is what you should expect from a bunch of charged particles following the magnetic lines of a dipole.
Buuuuuut, your question would have been stupid two decades ago. "Of course it's a circle!", because of dipole and shit. But look at Saturn; it's got an hexagonal polar "circle"-shaped storm. Turns out that standing waves matter. It may not affect auroras, but it's still pretty interesting to see something that's not circular happening at the poles of a planet.
They're circular because of the WAY they interact. I couldn't explain it, mostly because I'm don't quite know. But I know it only happens in a circle around the poles of a planet.
Buuuuuut, your question would have been stupid two decades ago. "Of course it's a circle!", because of dipole and shit. But look at Saturn; it's got an hexagonal polar "circle"-shaped storm. Turns out that standing waves matter. It may not affect auroras, but it's still pretty interesting to see something that's not circular happening at the poles of a planet.