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What is an Aurora?

Aurora, a phenomenon of the Arctic or Antarctic skies, known in the Northern Hemisphere as the Aurora Borealis, and in the Southern Hemisphere as the Aurora Australis.

The term Aurora Polaris is sometimes used to include both.

The Aurora usually occurs as an arc, or band, of white or colored light over the magnetic meridian, at a height of between 50 and 250 meters, though in a sun-illumined atmosphere the light has been traced up to a height of 600 m. It appears doubtful whether it extends below the clouds. Rays of light frequently stream from the arc and extend over the sky, either stationary or in a pulsating motion.

The Aurorae are electrical phenomena and there is ample evidence that a close relation exists between activities in the sun (sunspots) and their occurrence. Brilliant Aurorae are generally accompanied by magnetic disturbances, affecting telegraphic communications, with intervals of 17 to 25 hrs after direct observations of solar disturbances. Spectrum photographs of Aurorae show a typical green line due to oxygen, and spectrographs of the night sky have shown this line, too, suggesting the possibility of permanent Aurorae, invisible to the eye.

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