Aurora (astronomy)

Aurora (astronomy)

An aurora (plural: aurorae or auroras; from the Latin word aurora, "dawn") is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Aurora is classified as diffuse or discrete aurora. Most aurorae occur in a band known as the auroral zone, which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone will expand to lower latitudes. The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night. Discrete aurorae are usually observed only in the night sky because they are not as bright as the sunlit sky. Aurorae occasionally occur poleward of the auroral zone as diffuse patches or arcs (polar cap arcs), which are generally invisible to the naked eye.

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoctes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits". In Europe, in the Middle Ages, the auroras were commonly believed a sign from God.

Its southern counterpart, the aurora australis (or the southern lights), has almost identical features to the aurora borealis and changes simultaneously with changes in the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized.

• View of the aurora australis from the International Space Station

• False color DMSP image looking down from 850 km altitude showing discrete aurora (yellow) north of Scandinavia, clouds and fog are blue

• Red color is caused by nitrogen being bombarded with radiation from a solar flare

• Eielson Air Force Base, Alaska

Video of the aurora australis taken by the crew of Expedition 28 on board the International Space Station, its sequence of shots was taken September 17, 2011 from 17:22:27 to 17:45:12 GMT, on an ascending pass from south of Madagascar to just north of Australia over the Indian Ocean Video of the aurora australis taken by the crew of Expedition 28 on board the International Space Station, its sequence of shots was taken September 7, 2011 from 17:38:03 to 17:49:15 GMT, from the French Southern and Antarctic Lands in the South Indian Ocean to southern Australia Video of the aurora australis taken by the crew of Expedition 28 on board the International Space Station, its sequence of shots was taken September 11, 2011 from 13:45:06 to 14:01:51 GMT, from a descending pass near eastern Australia, rounding about to an ascending pass to the east of New Zealand