Noctilucent clouds seen over Vantaa in Finland in 2009.
© Mika-Pekka Markkanen
Noctilucent clouds seen over Vantaa in Finland in 2009
Tuesday marked the 10-year mission mark of a NASA satellite, named AIM for Aeronomy of Ice in the Mesosphere, whose purpose was to study noctilucent clouds and the data has proven invaluable.

Noctilucent clouds, also known as "night-shining" clouds, form on the edge of space about 50 miles high off the surface. They're made of ice crystals, which reflect sunlight to give off the clouds' signature blueish glow, according to NASA.

They're mainly seen in the summer just after sunset and before sunrise. Greg Johnson with caught a display over Hansville, Wash. last summer with his nighttime time lapse video camera:

The clouds are usually spotted about 30-60 minutes after sunset when the sun is between 6 and 16 degrees below the horizon, according to That's because the clouds are so high they can still "see" the sun from that altitude but it's dark enough on the surface to spot their cool, blue glow. Typically these clouds are brightest in late June and July.

Scientists had thought the clouds were actually the result of "meteor smoke" and it turns out, the satellite data confirmed they're on the right track.

"The accepted theory was that the ice formed around meteoric smoke — very small, nanometer-scale particles that are remnants of meteors burning up in the atmosphere," said Diego Janches, project scientist for the AIM mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "With AIM, we were able to study the presence and variability of that smoke."

The satellite's data have led to more than 200 papers on Earth's upper atmosphere. Some of the key discoveries over the past 10 years include contradicting earlier assumptions that the noctilucent clouds were tied to the sun's 11-year cycle and would rise and fall with sun activity. But not so -- the satellite shows the clouds have been steadily increasing over the past decade, NASA says. They're not exactly sure why, but scientists now suspect it has to do with increasing greenhouse gases.

Comment: Captain Obvious chimes in: maybe because there's more "meteor smoke"?!

"Combining AIM's data with 36 years of measurements from satellite instruments showed a correlation between more frequent noctilucent clouds and increases in water vapor, a greenhouse gas, and decreasing upper-atmosphere temperatures — a side effect of warming near the surface," wrote Sarah Frazier with NASA's Goddard Space Flight Center.

They've also been given more data to study how heat moves in the upper atmosphere, finding it's more linked to atmospheric circulation than direct heating form the sun, Frazier said. And they've also found the noctilucent clouds are likely responsible for what had been mysterious radar echoes picked up during the summer.

Next up for the satellite is to study gravity waves -- which are oscillations in the air usually caused by weather and winds near Earth's surface.

"These gravity waves affect the entire circulation of the middle and upper atmosphere," said Cora Randall, principal investigator of AIM's Cloud Imaging and Particle Size, or CIPS, experiment at University of Colorado Boulder. "These are really important for the global atmospheric structure and composition, and even affect the polar vortex."