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.
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.
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.