SKY TURNS GREEN, BUT WHY? On June 6th during a G1-class geomagnetic storm, the dark starry sky above Big Bend National Park in Texas turned green. It was not, however, the aurora borealis. Scroll past this picture of the phenomenon, taken by James and Karen Young, for an explanation:

"It's airglow," says Young. "We noticed it around 3 a.m. and it was quite bright."

Airglow is an faint bubble of everpresent light that surounds our entire planet, fringing the top of the atmosphere with aurora-like color. Although airglow resembles the aurora borealis, its underlying physics is different. Airglow is caused by an assortment of chemical reactions in the upper atmosphere driven mainly by solar ultraviolet radiation. Auroras, on the other hand, are caused by gusts of solar wind.

Green airglow is best photographed from extremely dark sites on nights when the Moon is new or below the horizon. It often shows up in long exposures of the Milky Way. Indeed, you can see the Milky Way and the Androdeda Galaxy in Young's photo, a 20 second exposure at ISO 6400. 

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