Aurora

An aurora (pl. aurorae or auroras) is a natural light display in Earth's sky, predominantly observed in high-latitude regions around the Arctic and Antarctic. The terms northern lights (aurora borealis) and southern lights (aurora australis) are used in the Northern and Southern Hemispheres respectively. Auroras display dynamic patterns of radiant light that appear as curtains, rays, spirals or dynamic flickers covering the entire sky.

Auroras are the result of disturbances in the Earth's magnetosphere caused by enhanced speeds of solar wind from coronal holes and coronal mass ejections. These disturbances alter the trajectories of charged particles in the magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere). The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles.

Other planets in the Solar System, brown dwarfs, comets, and some natural satellites also host auroras.

The term aurora borealis appeared a 1649 description by Pierre Gassendi of an auroral display visible all over France in 1621. Gassendi had read the works of Galileo Galilei who used the term in his extensive writings about aurora in 1619. The term entered the English language in 1828.

The word aurora is derived from the name of the Roman goddess of the dawn, Aurora, who travelled from east to west announcing the coming of the Sun. Aurora was first used in English in the 14th century. The words borealis and australis are derived from the names of the ancient gods of the north wind (Boreas) and the south wind (Auster or australis) in Greco-Roman mythology.

Auroras are most commonly observed in the "auroral zone", a band approximately 6° (~660 km) wide in latitude centred on 67° north and south. The region that currently displays an aurora is called the "auroral oval". The oval is displaced by the solar wind, pushing it about 15° away from the geomagnetic pole (not the geographic pole) in the noon direction and 23° away in the midnight direction. The peak equatorward extent of the oval is displaced slightly from geographic midnight. It is centred about 3–5° nightward of the magnetic pole so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun, which is called magnetic midnight.

Early evidence for a geomagnetic connection comes from the statistics of auroral observations. Elias Loomis (1860), and later Hermann Fritz (1881) and Sophus Tromholt (1881) in more detail, established that the aurora appeared mainly in the auroral zone.

In northern latitudes, the effect is known as the aurora borealis or the northern lights. The southern counterpart, the aurora australis or the southern lights, has features almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. The aurora australis is visible from high southern latitudes in Antarctica, the Southern Cone, South Africa, Australasia, the Falkland Islands, and under exceptional circumstances as far north as Uruguay. The aurora borealis is visible from areas around the Arctic such as Alaska, Canada, Iceland, Greenland, the Faroe Islands, Scandinavia, Finland, Scotland, and Russia. A geomagnetic storm causes the auroral ovals (north and south) to expand, bringing the aurora to lower latitudes or higher in the south. On rare occasions, the aurora borealis can be seen as far south as the Mediterranean and the southern states of the US while the aurora australis can be seen as far north as New Caledonia and the Pilbara region in Western Australia. During the Carrington Event, the greatest geomagnetic storm ever observed, auroras were seen even in the tropics.