They occur when solar activity in the form of highly charged electrons are blown towards earth in what is called “solar wind”. The electrons 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, they follow the lines of magnetic force generated by the earth's core and flow through the magnetosphere.
As the electrons enter the earth's upper atmosphere, they encounter atoms of oxygen and nitrogen at different altitudes, ranging from 20 to 200 miles above the earth's surface. The colour of the aurora depends on which atom is first struck and at what altitude, as follows:
• 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
It often looks like the auroras are dancing through the sky, this is because the magnetic and electric forces are reacting with each other in constantly shifting combinations.
This amazing spectacle is at its most active in the peak of sunspot activity which is an 11 year cycle. 2012, happened to be the near the peak, and thus was the perfect time to see some breathtaking displays- if you are lucky enough to be able to witness it firsthand, please take advantage; for everyone else, here is a link to some great photos: http://www.psdeluxe.com/
For an aurora forecast see here:http://www.gi.alaska.edu/