The Night Is Aglow With Natural Light, Too

On October 7, an astronaut aboard the International Space Station (ISS) shot this photograph while orbiting at an altitude of more than 250 miles  (400 kilometers) over Australia. You can see the Milky Way across the middle of the frame, the Andromeda Galaxy at top. Starlight and airglow combine to provide enough light to find your way around even in areas of zero light pollution. NASA

What an incredible number of stars. The photo was taken with a Nikon D5 digital SLR and 24 mm lens and shows the Earth at night from orbit, the Milky Way and even the Andromeda Galaxy, 2.5 million light years away. Don’t overlook that delicate, orange rind of light surrounding the planet. That’s airglow, a form of natural “light pollution.”

Coming in orange, green and red, these diffuse bands of light stretch 50 to 400 miles into our atmosphere. The phenomenon typically occurs when molecules  — mostly nitrogen and oxygen — are energized by ultraviolet (UV) radiation from sunlight. Atoms in the lower atmosphere are closely surrounded by other atoms and lose their extra energy by bumping into each other, but the upper atmosphere is thinner, with atoms less likely to collide. Those guys get rid of their extra juice by emitting light. The result is airglow.

Earth’s multi-colored glow

That said, some airglow is caused by collisions, when excited oxygen atoms smack and stick to form oxygen molecules (as in “O2″). In the video, the brightest green light in the first few seconds of the time-lapse is due to oxygen atoms that have recombined into oxygen molecules. Yellow colors are caused by emissions from a sodium layer. Meteoroids entering the atmosphere and burning up are the source of the sodium.

Green streaky airglow seen from the ground from a dark sky site in July 2014. While it shows up well in photos, the color’s not apparent to the eye. But it is visible, appearing as pale streaks of soft, diffuse light in an otherwise clear sky. It’s most apparent 15°-30° (1.5 to 3 fists) above the horizon. Any direction is good. Airglow is most intense in the daytime but invisible because it’s completely overwhelmed by daylight. Bob King

Other reactions can produce red, blue, UV, and infrared light. Red airglow shows up clearly at a minute-fifteen into the video,

“Airglow is a great tool for scientists because it reveals some of the conditions of the upper atmosphere, like its temperature, its shape, and the amounts of different types of gases,” said Sarah Jones, a NASA research astrophysicist.

Satellites offer one way to study this dynamic zone. NASA’s Ionospheric Connection Explorer (ICON) satellite, scheduled for launch in late 2018, will help scientists understand the physical processes at work where Earth’s atmosphere interacts with near-Earth space.

Green and red airglow layers stand out clearly in this photo taken from the space station. The brightest form of airglow is the lower green layer (~62 miles / 100 km) from excited oxygen atoms; the red layer is about twice as high and also forms when excited oxygen atoms dump their energy. City lights are visible on Earth’s nightside. NASA

To see airglow yourself, make a trek to a dark, rural sky with no moon. Now through about Nov. 9 is a good time because the moon’s on its way out of or gone from the evening sky. Allow your eyes at least 20 minutes to get accustomed to the darkness then do broad sweeps of the sky, looking for faint, grayish streaks or patches. Airglow is most obvious between 15° and 30° above the horizon, where our line of sight passes through a good quantity of the atmosphere, allowing the glow to “stack up” and appear more intense. A digital single-lens reflex camera (SLR) able to take 30-second time exposures at an ISO of 1600 and higher can help you track down the glow. Aim and photograph a section of sky then flick on the preview screen to see if and where you’ve captured it.

Airglow is different from the aurora, which is caused by particles (not radiation) from the sun beating on atoms in the upper atmosphere, but both phenomena trace their origin to our star.

4 Responses

  1. kevan hubbard

    Is it possible that I can spy m32 or m110 off m31 or is it just my eyes and a foreground star in our Galaxy?m32/110 are certainly tricky for down here on Earth although I think my 10×42 monocular nabs them?

    1. astrobob

      Hi Kevan,

      It’s much too faint — magnitude 8. You might be able to spot it in your monocular if you know exactly where to look.

  2. Mike Brown

    In that 10/7 photo – how long of an exposure do you think that is? I curious as to what they actually see naked eye. It can’t be too long of an exposure as the lights on the earth aren’t making trails.

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