This occures when when highly charged electrons from the solar wind interact with elements in the earth's atmosphere. Solar winds stream away from the sun at speeds of about 1 million miles per hour. When they reach the earth, some 40 hours after leaving the sun, they follow the lines of magnetic force generated by the earth's core and flow through the magnetosphere, a teardrop-shaped area of highly charged electrical and magnetic fields. The color of the aurora depends on which atom is struck, and the altitude of the meeting.
- Green - oxygen, up to 150 miles in altitude
- Red - oxygen, above 150 miles in altitude
- Blue - nitrogen, up to 60 miles in altitude
- Purple/violet - nitrogen, above 60 miles in altitude
If they're combined this means this means that there are all different kinds of altitude and nitrogen/oxygen mix. They go on an 11 year cycle and the next predited time is in 2011 or 2012, which is this year or next year. This is what I learned on my trip to Alaska for my 16th birthday. I hope that this can help. :)