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These curtains of coloured light that appear in the sky, predominantly in the Arctic and Antarctic regions of the earth, are known as the Aurora Borealis or Northern Lights (and Aurora Australis or Southern Lights).
The aurora borealis (the Northern Lights) and the aurora australis (the Southern Lights) have always fascinated mankind, and people even travel thousands of miles just to see the brilliant light shows in the earth's atmosphere.
In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas by Pierre Gassendi in 1621. The aurora borealis is only visible in the sky from the Northern Hemisphere, the chance of visibility increasing with proximity to the North Magnetic Pole, which is currently in the arctic islands of northern Canada.
The northern lights have had a number of names throughout history. The Cree people call this phenomenon the "Dance of the Spirits." Aurorae can be spotted throughout the world. It is most visible closer to the poles due to the longer periods of darkness and the magnetic field.
Aurorae seen near the magnetic pole may be high overhead, but from further away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the sun was rising from an unusual direction. The aurora borealis most often occurs from September to October and from March to April.
Benjamin Franklin first brought attention to the "mystery of the Northern Lights." He theorized the shifting lights to a concentration of electrical charges in the polar regions intensified by the snow and other moisture.
The auroras, both surrounding the north magnetic pole (aurora borealis) and south magnetic pole (aurora australis) occur when highly charged electrons from the solar wind interact with elements in the earth's atmosphere. Solar winds stream away from the sun at speeds of about 1 million miles per hour. When they reach the earth, some 40 hours after leaving the sun, they follow the lines of magnetic force generated by the earth's core and flow through the magnetosphere, a teardrop-shaped area of highly charged electrical and magnetic fields.
As the electrons enter the earth's upper atmosphere, they will encounter atoms of oxygen and nitrogen at altitudes from 20 to 200 miles above the earth's surface. The color of the aurora depends on which atom is struck, and the altitude of the meeting.
* Green - oxygen, up to 150 miles in altitude
* Red - oxygen, above 150 miles in altitude
* Blue - nitrogen, up to 60 miles in altitude
* Purple/violet - nitrogen, above 60 miles in altitude
aurora_borealis.jsp Source: www.geocities.com/abaccola/aurora.html