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An aurora seen over the South Pole, from the ISS. Credit: Doug Wheelock, NASA.
Also known the “northern” and “southern lights”, Auroras are natural light displays that take place in the night sky, particularly in the Polar Regions. They are the result of interaction in the ionosphere between the sun’s rays and Earth’s magnetic field. Basically, the solar wind launched by the sun contains clouds of plasma, full of particles that include electrons and positive ions. When they reach the Earth, they interact with the Earth’s magnetic field, exciting oxygen and nitrogen in the Earth’s upper atmosphere.
During this process, ionized nitrogen atoms regain an electron, and oxygen and nitrogen atoms return from an excited state to ground state. Excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule. Different gases produce different colors of light. Light emissions coming from oxygen atoms as they interact with solar radiation appear green or brownish-red while the interaction of nitrogen atoms cause light to be emitted that appears blue or red. This dancing display of colors is what gives the Aurora its renowned beauty and sense of mystery.
In northern latitudes, the effect is known as the Aurora Borealis, named after the Roman Goddess of the dawn, Aurora, and the Greek name for the north wind, Boreas. It was a French scientist named Pierre Gassendi who gave them this name after first seeing them in 1621. In the southern latitudes, it is known as Aurora Australis, Australis being the Latin word for “of the south”. 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.
The northern lights have had a number of names throughout history and a great deal of significance to a number of cultures. The Cree call this phenomenon the “Dance of the Spirits”, believing that the effect signaled the return of their ancestors. To the Inuit, it was believed that the spirits were those of animals. Some even believed that as the auroras danced closer to those who were watching them, that they would be enveloped and taken away to the heavens. In Europe, in the Middle Ages, the auroras were commonly believed to be a sign from God.
The aurora is usually best seen in the Arctic and Antarctic because that is the location of the poles of the Earth’s magnetic field. Auroras can be spotted throughout the world and on other planets. They are most visible closer to the poles due to the longer periods of darkness and the magnetic field.
We have written many articles about the Aurora for Universe Today. Here’s an article about the Aurora Borealis, and here are some views of the Aurora from space.
If you’d like more info on the aurora, check out Space Weather Center, and here’s a link to some amazing Aurora Borealis pictures taken from Alaska.
We’ve also recorded an episode of Astronomy Cast all about Aurora. Listen here, Episode 163: Auroras.
Sources:
http://www.cbc.ca/technology/story/2010/08/03/solar-storm-aurora-borealis.html
http://en.wikipedia.org/wiki/Aurora_(astronomy)
http://scienceray.com/astronomy/dance-of-the-spirits/
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