Lightscape / Night Sky
Image courtesy JAcob W. Frank
Solar winds stream away from the sun at speeds of about 1 million miles per hour and reach the earth roughly 40 hours after leaving the sun. As the electrons enter the earth's upper atmosphere, they will encounter atoms of oxygen and nitrogen at altitudes from 20 to 200 miles above the earth's surface. 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
All of the magnetic and electrical forces react with one another in constantly shifting combinations. These shifts and flows can be seen as the auroras "dance," moving along with the atmospheric currents.
The auroras generally occur along the auroral ovals, which center on the magnetic poles and roughly correspond with the Arctic and Antarctic circles. The lights can be visible at lower latitudes when solar activity is high.