The northern lights are the spectacular flickering curtain of lights seen dancing against a darkened sky. The northern lights, called the aurora borealis in the northern hemisphere, have been a colorful feature of earth’s night sky since the earth acquired an atmosphere.
Since the mechanism of the aurora was not scientifically understood until relatively recently, numerous interesting myths were passed along to explain the mysterious light events. One Scandinavian myth attributed the aurora to vast fires surrounding the ocean, and another to the reflections in the sky caused by large swarms of herring in the sea. Some Native Americans saw in the aurora their ancestors gathered around a ceremonial fire, and the Inuit of the north believed the aurora to be spirits playing with a walrus.
The scientific explanation for the action of the aurora involves the solar wind, the earth’s magnetic sphere, charged particles, and atmospheric gases. Activity on the sun powers the aurora activity, and since the sun’s activity increases and decreases periodically, so does the visibility of aurora activity on earth increase and decrease.
The solar wind is a stream of negatively charged electrons and positively charged ions propelled into space by the power of the 15 million degree heat of the sun’s interior and the sun’s magnetic field. The charged particles travel as a plasma through space in all directions at velocities up to 1000 kilometers per second, reaching the earth at a velocity of around 400 km/s.
Without the protective atmosphere and the magnetic field surrounding the earth the solar wind would make the earth uninhabitable. The atmosphere effectively deflects the solar wind around the earth but the magnetic tail of our magnetic field brings some of the particles back to earth and directs them to the magnetic polar north and south. The charged particles then accelerate again, propelled by our own magnetic field. As the charged particles, negative electrons and positive ions, enter the earth’s ionosphere at the poles they collide with and excite the molecules of the atmospheric gases. The collision with the various gas molecules causes them to emit photons of light. Different gases in the ionosphere emit different wavelengths, which account for the different colors that are often seen in an aurora display.
In addition to different colors the aurora can exhibit numerous different structures. For example, the aurora may look like a very lengthy arc or band of light with a certain width. On other occasions the aurora may look like a band with rays emanating upward from the band. Alternately, the pattern may exhibit all rays, dancing, expanding, and contracting. The length of the rays may be hundreds of kilometers tall. Perhaps most spectacularly, the aurora pattern may be a huge curtain of light rays filling the sky in all directions, with colors and intensities varying rapidly.
The aurora in the southern hemisphere is identical to the aurora in the north, but in either hemisphere the aurora can best be seen close to the poles. During periods of intense aurora activity, usually coinciding with periods of increased sun spot storms on the surface of the sun, the dancing northern lights become visible farther from the poles.
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