For the past week, solar wind has been buffeting Earth's magnetic field, turning skies around the Arctic Circle beautiful shades of green. But not every green sky is caused by the aurora borealis. Last night, for example, pilot Brian Whittaker was flying 34,000 feet over the Atlantic Ocean when he witnessed verdant hues caused by a completely different phenomenon--airglow. Here is the picture he took from the cockpit window:
Airglow
© Brian Whittaker

"A dark and moonless night away from all lights allowed a great view of this textured patch of airglow," says Whittaker. "The illumination was faint, but it could be seen especially in contrast to the dark ocean abyss below!"

Although airglow resembles the aurora borealis, its underlying physics is different. Airglow is caused by an assortment of chemical reactions in the upper atmosphere. During the day, ultraviolet radiation from the sun ionizes atoms and breaks apart molecules. At night, the atoms and molecules recombine, emitting photons as they return to normal. This process produces an aurora-like glow visible on very dark nights.

Because the Moon is new on Sept. 15th, tonight is a good night to spy this phenomenon. Get away from city lights, if you can, and take a look!