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Space Station Science
Picture of the Day

April 9, 2003

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-shaded
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.

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