The color varies, depending how hard the gas particles are being struck.  Auroras can range from simple arcs to drapery like forms in green (the most common), red, blue, and purple.  The lights occur in a pattern rather than a solid glow, because electric current sheets flowing through the gases create a V shaped potential double layers.

These beautiful displays take place as low as 40 to 68 miles above the earth's surface and extend hundreds of miles into space.  The lights concentrate in two bands centered above the Arctic Circle and Antarctic Circle, and are about 2,500 miles in diameter.  The southern lights are known as the Aurora Australis.  In the northern latitudes the greatest occurrence of aurora displays is in the spring or fall months, owing to the tilt of the earth to the sun.  The first 1982 published photographs of the northern lights in their entirety were taken from satellite cameras and had the appearance of a nearly perfect circle.