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That green glow behind the mountains is not always the Northern Lights

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Photo: Meg McDonald, Wild Northwest Beauty Photography

The clear skies this summer had photographers racing to the mountains to get dramatic photos of our Milky Way Galaxy core, but that's not all they captured.

Some photos will show a tinge of green or teal in the background -- but it's not the Northern Lights.

Instead it's known as "air glow."

Astrophotographer Steven Rosenow explains what causes it:

"While aurora and airglow can be easily mistaken for one another, the two mechanisms which create the two are completely different. Aurora occurs as a result of solar wind and/or solar magnetic storms interacting with Earth's magnetosphere. Airglow, on the other hand, is akin to shining a light on a glow-in-the-dark object. In this case, the sun is the light, and our planet's mesosphere and ionosphere is the 'glow-in-the-dark' object. Solar light charges the ionosphere and at night, oxygen and nitrogen react with hydroxyl ions, creating a phenomenon known as chemiluminescence."

So unlike aurora, airglow covers the entire planet and is there all the time, caused by reactions between solar energy and certain air molecules. It's just not visible in daytime due to sunlight and is hard to spot at night from other light pollution.

But it wasn't hard to spot with Meg McDonald's camera in late July.

"The airglow over Olympic National Park is the clearest and most beautiful display that I’ve ever photographed," she said. "In both of my videos I included some of my photographs of the stunning national parks that were sleeping under the stars in my time lapse sequences. The night sky is incredible in itself, but knowing the beauty that’s beneath those stars makes watching the majesty of the night sky even more meaningful."

Some of the streaks visible on her photos are not meteors, but Iridium flares -- essentially sunlight reflecting off the mirrored surfaces of Iridium communication satellites as they orbit the planet.

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