Dec. 10, 2007 -- Ice clouds at the edge of space are bigger, more prevalent and surprisingly variable, say researchers who believe the phenomenon may be partly due to global climate change.
The findings stem from observations by NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite, which studies noctilucent or "night-shining" clouds 50 miles above the polar regions.
The clouds are visible by night because they are so high above Earth that they are illuminated by sunlight while the ground below is in darkness. They appear during the summer months over both the north and south poles.
"These clouds are changing in ways that we don't understand," AIM principal investigator Jim Russell, with Virginia's Hampton University, said Monday at the opening of the American Geophysical Union Conference in San Francisco.
Researchers believe the clouds are more numerous and denser than expected due to changes in Earth's climate.
"The clouds are an exquisite thermometer," said Virginia Polytechnic Institute's Scott Bailey, deputy principal investigator for AIM.
Scientists have been keeping tabs on the clouds for about 27 years through a series of solar-watching satellites. AIM is the first mission devoted to studying the clouds exclusively.
The clouds appear more frequently than in the past and form at lower altitudes, which, Russell said, may be due to a buildup of carbon dioxide in the lower atmosphere. The gas traps heat and leaves the upper mesosphere colder than normal.
The scientists believe a difference of just 5 degrees Fahrenheit has profound effects on the clouds' structure and motion.
"A small change in temperature is causing a dramatic change in cloud behavior," Bailey said.
During its first season of operations, AIM confirmed a long-held suspicion that a summertime radar phenomena observed at the polar regions was due to signals bouncing off ionized ice crystals in the upper atmosphere.
Scientists never expected to see so much variation in the clouds.
"It's the changes day-to-day that really surprised us," Bailey said. "Even from orbit to orbit, we can see differences in the behavior of the clouds."
Scientists, for example, tracked a temperature change over five days and found corresponding changes in a cloud's location.
Analysis of the chemical composition of the clouds is under way.
Among the results of AIM's first observational season:
*The clouds appear daily, are widespread and change on an hourly and daily basis.
*The clouds are about 10 times brighter than expected based on previous measurements and vary within horizontal spans of about two miles.
*Ice in the upper atmosphere extends in a continuous layer from about 51 miles to 55 miles above the planet's surface.
*The clouds share characteristics associated with normal tropospheric clouds, suggesting that some of the same dynamics that trigger the planet's weather operate high above Earth as well.
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