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The sun is continually sending out particles (mostly protons and
electrons) into space; some are blasted out by solar flares and sunspots, others
"boil" off from the very hot corona (the sun's upper atmosphere) into the solar
system. This is called the solar wind. The earth's magnetic field shields the ea rth from most of
these particles; some get trapped in doughnut-shaped regions surrounding the
earth called the Van Allen belts. Particles continually "leak" from these
regions, traveling down along the earth's magnetic field lines (that's the reaso n they are usually seen only
near the north and south poles), until they crash into the earth's upper
atmosphere. The collisions transfer energy to atoms of gas, which then give off
light, just like a neon sign. Pictures from space show that there is a permanent "ring" of auroral
activity around the poles; but during times when the sun is very active,
northern auroras may be seen as far south as Mexico.
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