The spectacular green banding seen in the Devils Island photo at first glance look like aurora borealis (northern lights), but in fact is something different. It's called banded air glow.

Even absent light from the stars or other celestial objects, the night sky is never completely dark. Earth's atmosphere itself glows faintly all the time as atoms of oxygen, having been excited during daytime hours by UV radiation from the sun, emit faint light as they drop back to their less energized state.

On the night we shot this photograph, this air glow display was unusually bright. To the unaided eye, it looks like a very thin cloud layer, and usually escapes notice altogether. Only with the exceptionally dark skies of the Lake Superior region and the long exposure capability of the camera did the color emerge. The banded structure is caused by atmospheric gravity waves, a common wave-like movement of air high in the thermosphere (more than 50 miles up).

Details of the shot:
Location: Devils Island, Apostle Islands National Lakeshore
Date: July 20, 2012, 12:20 am
Camera: Canon 7D
Specifics: 4 minute exposures (2 shots, telescope drive on, telescope drive off), ISO 2000, Canon 10-22mm lens operating at 10mm at f/4.