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What is an auroral oval?

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Diagram of Earth's atmosphere layers-troposphere, stratosphere, mesosphere, and thermosphere-showing how temperature and air pressure change with increasing altitude, with key features like the ozone layer and pressure percentages at various heights. — Ionosphere and magnetosphere The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.

An auroral oval is an oval-shaped region surrounding Earth’s magnetic poles where auroras typically occur. These regions maintain a relatively fixed orientation with respect to the Sun, though their exact position can shift based on solar activity.123

The existence of these ovals is explained by the topology of magnetic field lines resulting from magnetic reconnection. The boundaries of the auroral oval are defined by the projection of specific magnetic field lines onto the polar atmosphere:43

Poleward boundary: Formed by the projection of the "last-closed field lines" on the nightside, which separate regions open to the solar wind from those that are closed.4
Equatorward boundary: Formed by the inner edge of the plasma sheet, where charged particles drift earthward under the influence of magnetospheric convection.4

The center of the auroral oval is typically displaced slightly toward the nightside relative to the geomagnetic pole. During periods of intense solar activity, these ovals can expand, causing auroras to be visible at lower latitudes.12