Aurora borealis, Whitehorse, Yukon Territory, Canada. Copyright Paul Nicklen/National Geographic Image Collection

quiet auroral arc in the midnight sector. This brightening spreads westward along the auroral oval, and then the aurora expands poleward (termed the expansive phase of the substorm). During this time, the aurora is most active and colorful, with draperies, transient rays, and rapidly moving arcs. After this explosion of activity and color, which may last up to 20 min, the aurora fades and recedes to lower latitudes and is replaced by fainter patches, often pulsating with a period of a few seconds. This recovery phase lasts for up to 2 h.

Great auroras expanding to low latitudes and lasting up to two days occur very occasionally, and have been marveled at through the ages. They are marked by their unusual brightness, near-global extent, and long duration. Global power inputs via particle precipitation have been estimated as high as 1000 GW during the peak of such auroral displays. They tend to occur at, or following, the peak of the 11-year cycle of solar sunspot activity. Some such recent great auroras were the February 10-11, 1958 display, which drove instruments off-scale and was seen as far south as New Mexico, one on March 13-14, 1989, which caused major power disruptions along eastern North America, and the November 8-9, 1991, aurora which was first observed at the magnetic pole and gradually extended down to midlatitudes. A January 10-11, 1997 display viewed from the magnetic pole down to the middle United States and Europe was noteworthy in that its full evolution was recorded from "the cradle to the grave" by well-located spacecraft. The orbiting Solar and Heliospheric Observatory (SOHO) observed the solar CME, which caused it. The progress of the cloud and the solar wind streaming earthward were monitored by the WIND satellite, and the aurora resulting some 80 h later (the Sun-Earth transit time) was photographed by the POLAR satellite. While there may be something special causing these very unusual auroras, evidence so far suggests that they are just much bigger than the usual substorms and not very different.

There have been written accounts of auroras through the ages as far back as nearly 600 BC, with explanations often invoking heat and fire. Folklore emerged as early peoples sought to explain the unique phenomenon in familiar terms. Scandinavians often associated the northern lights with wildlife—swans caught in the ice, a fox running across mountains, whales threshing through waves, or reflections from fish. Most Inuit attach spiritual significance to the lights, believing them to represent the souls of the dead that are waiting to be reborn. To the Ottawa Indians the aurora was the Creator igniting the skies to see how his peoples were faring. Perhaps more generally it was "the gods dancing across the firmament," with the Scots describing the northern lights as the Merry Dancers. But to many societies aurora evoked fear—that it could descend and grab up children or the old, or that it presaged impending disasters such as fires, plagues, or wars. Early literature abounds with picturesque or fanciful outpourings of feelings inspired by auroras.

Polar auroras, first noted by early Arctic explorers, are of similar origin to lower altitude auroras but are somewhat different in character. They are seen poleward of the auroral zone and occur only under quiet magnetic conditions when auroral oval activity is minimal. They consist of very narrow arcs, usually faint and always aligned along the Sun-Earth line. They are usually seen to split off the poleward edge of the auroral oval and drift across the polar cap, or linger for hours, depending on the state of the solar wind.

Auroras occur both in the Arctic and Antarctic and are near identical, according to simultaneous observations. In northern latitudes they are called aurora borealis or northern lights, while in the south they are called aurora australis. This symmetry is due to the Earth's magnetosphere and the manner in which it deflects and traps the solar wind particles that produce auroras. Auroras occur on other planets that have a magnetic field and an atmosphere. They are observed on Saturn and Jupiter.

Donald J. McEwen See also Space Weather; Substorms

Further Reading

Akasofu, S.-I., Aurora Borealis: The Amazing Northern Lights,

Anchorage: Alaska Geographic Society, 1979 Brekke, Asgeir and Alv Egeland, The Northern Lights, Their Heritage and Science, translated by James Anderson, Oslo: Grondahl og Drewers Forlag AS, 1994 Eather, Robert, Majestic Lights, The Aurora in Science, History and the Arts, Washington: American Geophysical Union, 1980

Savage, Candace, Aurora, The Mysterious Northern Lights, Vancouver: Greystone Books, 1994


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