The Aurora Borealis in the northern hemisphere (and the Aurora Australis in the southern hemisphere) is one of nature's most spectacular events, perhaps even more inspiring than the St. Thomas Carnival fireworks. The photo shown is but one of many different forms that are seen. The display may range from giant flowing blue curtains to crimson arcs. You can see a gallery of the many different types in photographs taken by Jan Curtis. (You can also see an aurora from space. )
Although the Aurora Borealis can, on rare occasions, be seen as far south as middle America, typically it is seen only in the far northern regions. There is a reason for this. The aurora is the result of charged particles from the sun being trapped by the earth's magnetic field. The earth's field resembles that of a bar magnet. The field lines emanate from near the geographic south pole and flow back into the earth near the geographic north pole. (These locations are called the geomagnetic north and south poles respectively.) The force exerted on moving charged particles is perpendicular to the velocity. In class, we saw how for a uniform field this results in motion along a cylindrical spiral. However, the earth's field strength increases toward the poles. As a result, the charged particles move in an ever-tighter spiral as they head toward the poles and into the earth's atmosphere. (Click on the picture for info on the earth's magnetosphere, a region of magnetic and electric fields created by the earth's magnetic field and the solar wind. )
It is the collision of the charged particles with atoms in the earth's atmosphere that produces the light. Electrons in the atoms are physically bumped into higher quantum energy states. (We will study this in some detail near the end of the semester.) It is when the electrons fall back to their original energy states that light is emitted. This happens primarily near the earths' poles, so you need to be near the poles to see the aurora. You can get a general layman's description of how this works from the Exploratorium in San Francisco. A far more detailed multiple-page tutorial is provided in The Straight Scoop , from the Poker Flat Research Range at the University of Alaska. This link actually starts in the middle of the tutorial. You may wish to explore other parts of the site. Clicking on the index button will take you to a subject list. A summary description is also provided.
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