As the solar wind reaches earth, some of the charged particles move into the planet's atmosphere. As they do so, they begin to encounter the
gases that are enclosed within. Molecular oxygen (O2) fluoresces with green light up to 150 miles from the earth's surface, and is the most common
molecule struck by these high energy electrons. Molecular nitrogen (N2) fluoresces with blue light up to 60 miles in altitude , and atomic oxygen
fluoresces with red light above 150 miles in altitude. These gases emit light at these altitudes because the electrons moving into the earth's
atmosphere will not be able to move past (because the molecules become so dense) without striking these gases. When these gases fluoresce,
they form the aurora that we know and love. Aurorae can form on any planet with a magnetic field. The color of these aurorae depend on the
gases held within the planet's atmosphere. For example, while our planet usually has a green aurora (from electrons colliding with O2), planets
with high levels of nitrogen in their atmosphere (such as Jupiter) will have a blue aurora.
http://science.howstuffworks.com/nature/climate-weather/atmospheric/question471.htm http://earthobservatory.nasa.gov/IOTD/view.php?id=6226 odin.gi.alaska.edu/FAQ
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