Florida's city lights shine brightly in this night view from the International Space Station — but there's a completely different kind of glow that frames the edge of our planet. It's known as "airglow," the faint greenish radiance high up in Earth's atmosphere.
You might think airglow comes from the reflected glare of city lights, but it's actually a photochemical reaction caused by the sun's ultraviolet radiation.
During the day, that radiation breaks apart molecules of oxygen and other chemicals, exciting them into a higher energy state. During the night, the molecules recombine, radiating the excess energy as light. As a result, the atmosphere glows in a thin region around 60 miles up.
The glow is so faint that it can't be seen when you're looking up into the sky, or when astronauts are looking directly down from space. But when space travelers look toward the edge of Earth's disk at night, they can see the permanent aurora at the horizon. For more about airglow, check out this explanation from the University at Albany's Bob Keesee, or this one from Discovery News' Jason Major.
We've seen some great views of airglow from the space station over the past year: For more examples, check out these archived PhotoBlog items, and keep an eye on NASA's Gateway to Astronaut Photography of Earth as well as astronaut Ron Garan's postings on Google+ for "Fragile Oasis."
The hits just keep on coming from the space station, and you can expect to see much more when the station's crew is back at its full strength of six astronauts.
Three new crew members are due to arrive this week, just in time for Christmas. In this video, space station commander Dan Burbank reflects on the new arrivals, the holidays and our "indescribably beautiful" planet:
The space station crew's sidelong glance at the Florida Keys, the Florida Peninsula and the rest of the southeastern United States was captured from orbit on Nov. 24. It's today's offering from the Cosmic Log Space Advent Calendar, which highlights views of Earth from space every day from now until Christmas. Catch up on these previous gems from the calendar:
- The full Cosmic Log Space Advent Calendar
- Dec. 1: An ornament in outer space
- Dec. 2: The masses in Mecca
- Dec. 3: Santa's shrinking domain
- Dec. 4: The monster of Madagascar
- Dec. 5: Antarctica stripped naked
- Dec. 6: Streaking for home
- Dec. 7: Pearl Harbor from above, 1941-2011
- Dec. 8: The rise and fall of the Dead Sea
- Dec. 9: How an eclipse dims Earth
- Dec. 10: Psychedelic storm
- Dec. 11: Beauty of the Inland Sea
- Dec. 12: Drone-spotting stirs up debate
- Dec. 13: Light up your St. Lucy's Day
- Dec. 14: Satellite spots Chinese aircraft carrier
- Dec. 15: Hooray for Hollywood
- Dec. 16: Olympics under construction
- Dec. 17: Mystery in the Gobi Desert
- Hubble calendar, from The Atlantic's In Focus
- 2011 Zooniverse Advent calendar
Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.