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Why do auroras usually appear in oval regions around the geomagnetic poles?

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geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

Auroras typically appear in oval regions around the geomagnetic poles because of the configuration of Earth’s magnetic field.12

Charged particles—primarily electrons and protons—from the solar wind are guided by the planet's magnetic field lines. Because these field lines originate and converge at high latitudes near the magnetic poles, the charged particles are funneled toward these regions. As these particles travel along the bundled, ropelike magnetic field lines, they intercept the upper atmosphere, where they collide with atoms of oxygen and nitrogen, causing the luminous emissions known as auroras.342

The specific oval shape of these zones is determined by the projection of the magnetosphere's boundaries onto the atmosphere. The nightside auroral oval corresponds to the projection of the plasma sheet and the "last-closed" magnetic field lines, while the dayside oval is associated with the projection of polar cusps where magnetic field lines split.56