Aurora

An aurora is light produced when energetic particles excite atoms and molecules in a planet's upper atmosphere. On Earth, the displays occur mainly in high-latitude regions and often appear as arcs, bands, curtains or rays. In the Northern Hemisphere they are called the aurora borealis, or northern lights; in the Southern Hemisphere they are the aurora australis, or southern lights. Their zones form broad ovals centered on the geomagnetic poles rather than on the geographic poles.

The Sun continuously sends charged particles into space as the solar wind, and eruptions such as coronal mass ejections can disturb it. When solar activity interacts with Earth's magnetosphere, energy is transferred into near-Earth space. Electrons there can be accelerated along magnetic field lines toward the upper atmosphere. Their collisions with atmospheric atoms and molecules raise those particles to higher energy states; as they return to lower states, they release photons, which make the aurora visible. Thus the Sun helps power auroral activity, while the Earth's magnetic environment and upper atmosphere shape how it appears.

Auroral colors depend on which gases are excited and on the conditions and altitude at which the emissions occur. Oxygen commonly produces green light and can also produce red; nitrogen contributes blue, purple and pink hues. Colors can blend, while changing particle input and geomagnetic conditions make the displays shift in brightness and structure. The familiar moving curtains and arcs are part of a much larger, changing auroral oval, not a fixed band of lights over one place.

Auroras are usually most readily seen from high latitudes near the Arctic and Antarctic regions. During stronger geomagnetic disturbances, the auroral ovals expand and displays can extend toward lower latitudes. The same space-weather disturbances that accompany bright auroras can affect systems that rely on radio signals, satellite operations, navigation and electric power grids. The auroral light itself is not the cause of those effects; it is a visible sign of changes in the coupled Sun–Earth environment.

Earth is not the only planet with auroras. They have also been observed on Jupiter and Saturn, where energetic particles interact with upper atmospheres in the presence of planetary magnetic fields. Because each world has different atmospheric gases and magnetic conditions, its auroral displays provide scientists with a way to study processes that occur throughout a planetary system.