Why They Are Different Colors     The different colors you see in an aurora depend on the type of gas that is being excited as well as the altitude and thus density of the air. Oxygen atoms give off deep reds at altitudes of 100-150 miles from the ground and green whitish colors when they're closer. Nitrogen gives off blue and red colors when it is 80-100 miles from the ground. There are other gases in the atmosphere that give off different colors such as neon, helium or hydrogen, but are only present is small amounts so are more difficult to detect. Knowing the colors and their respective atoms, you can look at an aurora and see where there must be a lot of oxygen atoms or nitrogen and where different elements are mixed. Courtesy of http://www.exploratorium.edu/learning_studio/auroras/ Photo by Jan Curtis Here you can see the this aurora has oxygen in it where at the high altitude it is red and near the ground it begins to have a green color. Photo by Jan Curtis In this picture you can see the oxygen on top with the green white glow. The color on the bottom is from nitrogen that has mixed its blue and red colors together to get a nice purple.

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