PLANET AIRGLOW: Jeff Dai was camping in the Himalayas on Nov. 18th when he photographed a green light in the sky. But it was not the aurora borealis. “It was airglow,” says Dai, who wrapped his photos of the display into a ‘little planet’:
“This is a 360-degree stereographic projection showing our camp site in Tibet,” says Dai. “In places like this, far from urban lights, the sky is filled with wonders.”
Although airglow looks like the aurora borealis, it is not. Auroras are caused by gusts of solar wind. Airglow is caused by chemi-luminescent reactions in Earth’s upper atmosphere. These reactions get started during daylight hours when the atmosphere is bathed in strong UV radiation from the sun. At night we see the afterglow, colored green by oxygen atoms 90-100 km high or sometimes red by hydroxyl ions 86-87 km high.
“Also unlike auroras, airglow is visible all over the globe,” adds Dai. “Though brightest 10-15 degrees above the horizon it fills the sky and may be photographed from dark-sky sites everywhere.”