A brilliant green section of the aurora borealis in its arc form shines over the skies of the Alaskan wilderness, a prime spot for aurora watching. This aurora occurred in the fall, when sky-watchers often report seeing auroras. Scientists don't yet know why, but geomagnetic storms that ignite auroras tend to happen more frequently during the months surrounding the fall and spring equinoxes. Green auroras like this one are the result of energized oxygen located 150 miles or less above Earth's surface.

This image is part of the PBS Nova documentary Magnetic Storm . Click here to watch.