Aurora Borealis: A Brief Overview

a band of green aurora tinged with a pink edge, arcing through a night sky over trees with golden leaves
Aurora, seen in Denali National Park

NPS Photo / Kent Miller

The aurora borealis (Northern Lights) occurs when a coronal mass ejection (CME), a massive burst of solar wind and magnetic fields, interacts with elements in the earth's atmosphere. Coronal mass ejections are often associated with other forms of solar activity, most notably solar flares. Near solar maxima the sun produces about three CMEs every day, whereas near solar minima there is about one CME every five days.

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.

Check out current aurora activity and the aurora forecast.

Last updated: September 19, 2017