April 9, 2003
Aurora Blues
Credit: Don Pettit, ISS Expedition
6 Science Officer, NASA
Explanation: Astronauts onboard the International
Space Station (ISS) have seen a lot of auroras in recent
months. They’ve even flown through some. Usually the lights are
green; sometimes they’re red. Other colors are rare. So when
ISS science officer Don Pettit looked out the window on March
29th and saw these striking blue-tinged auroras over Scandinavia,
he had to grab his camera and take a picture.
Auroras get their colors from
different atoms and molecules in Earth’s atmosphere. Greens and
reds come from atomic oxygen. Blues from molecular nitrogen.
Usually, though, nitrogen adds only a hint of blue–often unnoticeable.
What makes these Northern
Lights different?
"Perhaps it’s because they’re
sunlit," speculates NASA atmospheric physicist Joe Minow.
Off-camera the Sun is setting behind Earth’s limb. Although it’s
dark on the
ground below, the tops of these auroras are still illuminated
by sunlight. Some of that light is captured by ionized nitrogen
molecules (N2+), which are "tuned" by the
fundamental constants
of nature to absorb light at wavelengths of 391.4 and 427.8
nanometers—i.e.,
blue and violet. Blue sunlight absorbed by N2+ is
quickly re-emitted in all directions. "We call this process
resonant scattering of sunlight," notes Minow. "It
may be responsible for much of the blue in this picture."
Sunlit auroras are uncommon,
but Minow has seen them before from Alaska. "My wife always
put sunlit auroras into a class she calls ‘nerd auroras.’"
recalls Minow. "They’re usually faint and not very impressive
compared to the brilliant displays one sees during a major geomagnetic
storm. Only auroral physicists would consider them interesting!"
he laughs.
In many cases, however, what’s
dull on Earth looks extraordinary from
Earth-orbit. Add sunlit auroras to the list.
Photo Notes: Some stars are visible shining through
the auroras. The quartet of stars on the right outline the head
of the constellation Delphinus,
the dolphin. The stars in the center and left are from the constellation
Pegasus.
The smudged lights in the foreground are northern European cities.
Unlike the distant stars, the nearby city lights are noticeably
smeared by the orbital motion of the space station. | 
