Auroral oval

Viewed from space, auroral emissions form broad, uneven ovals around the northern and southern geomagnetic poles. The ovals are not centered on Earth's geographic poles, and their boundaries and brightness vary with time and magnetic activity. Although the displays are often associated with polar regions on maps, an aurora is a feature of the magnetosphere-atmosphere system, not a ring of light permanently attached to a particular surface location.

Interactions between the solar wind and Earth's magnetosphere transfer energy into near-Earth space and can accelerate charged particles along magnetic field lines. Electrons precipitating into the upper atmosphere collide with oxygen and nitrogen atoms and molecules, exciting them. As these particles return to lower-energy states, they emit photons that create auroral light. The auroral emissions map the regions where these particle precipitation processes occur; their oval form reflects the geometry and dynamics of the magnetosphere and its field lines.

The auroral oval is dynamic rather than a sharply bounded, permanent circle. During stronger geomagnetic activity, auroras generally become brighter and the oval expands toward lower geomagnetic latitudes, making sightings possible farther from the poles. During quieter conditions, the most active emissions tend to remain farther poleward. The oval also extends onto Earth's dayside, though sunlight can make aurora difficult or impossible to see from the ground there. Forecasts therefore describe likely activity and viewing regions rather than guarantee that a particular observer will see an aurora.

Earth has auroral ovals in both hemispheres, associated with the aurora borealis in the north and aurora australis in the south. Magnetic field connections can produce broadly corresponding, or conjugate, features, although the two hemispheres need not show identical patterns at every moment. Auroral ovals have also been observed at magnetized giant planets such as Jupiter and Saturn. Their auroras arise in planetary magnetospheres, whose plasma sources and interactions differ from Earth's.