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Last Updated: Tuesday, 11 December 2007, 13:30 GMT
Spacecraft chases highest clouds
By Jonathan Amos
Science reporter, BBC News, San Francisco

Noctilucent clouds  (Nasa)


Remarkable new images of Earth's highest and most mysterious clouds have been captured by a Nasa spacecraft.

Noctilucent, or "night-shining", clouds appear as thin bands in twilight skies, some 80km (50miles) above the surface.

The AIM probe has now returned the first truly global pictures of these phenomena which appear to be increasing in frequency and extent.

Scientists say their observations show how the clouds alter rapidly, hour by hour and day by day.

They hope their studies will reveal the key triggers to the clouds' formation and why these triggers appear to be undergoing long-term change.

AIM picture (Nasa)
AIM is getting the first truly global view of the unusual clouds

"These clouds are getting brighter with time, they're seen more often and also they're being seen at lower latitudes," said James Russell from Hampton University, Virginia, US.

"These are things we don't understand and they all suggest a possible connection to global change; and we need to understand that connection and what it means for the whole atmosphere," he told BBC News.

Dr Russell was speaking here at the American Geophysical Union Fall Meeting.

He is the principal investigator on the Aeronomy of Ice in the Mesosphere (AIM) mission, a 195kg (430lb) satellite that was launched in April of this year.

Sitting 600km above the Earth, the spacecraft has the perfect vantage point from which to study the clouds that are also sometimes referred to as Polar Mesospheric Clouds.

They form at high latitudes during summer months in an extremely cold (-160C), dry (100,000 times drier than the Sahara Desert), and low-pressure (100,000 times less than at the Earth's surface) environment.

Noctilucent clouds (BBC)
The changes in frequency and brightness have been observed over the past 30 years of satellite observations
They are normally apparent in summer months at high latitudes - at about 50-65 degrees north and south
They have been seen recently as low as 40 degrees North and have become a popular target for amateur photographers
One study implicated water in space shuttle exhaust plumes as a contributor - and AIM will be following up this theory

Dr Russell said AIM's three instruments had given a completely new perspective on the clouds.

"We'd never seen a picture of the whole polar region before, to see these clouds on a daily basis. That in itself is a revelation - it shows how variable they are from day to day, from orbit to orbit," he added.

From the AIM images, it is clear the locations where the clouds form seem to rotate around the Arctic with a period of about five days. That rotation in longitude is also seen in the temperature data.

"The interesting thing is that the magnitude of the temperature changes is only about five degrees Fahrenheit (3C)," said Dr Scott Bailey, AIM's deputy principal investigator from Virginia Polytechnic Institute and State University.

The AIM spacecraft (Nasa)
The AIM spacecraft looks over and through the clouds
"So, a very small change in temperature leads to a dramatic change in cloud behaviour. We conclude from that that these clouds are a very sensitive measure of temperature change."

AIM's data also shows the clouds to be much brighter than was previously thought. There are unprecedented observations, too, of the precursors to the clouds - small ice crystals just 5-8 nanometres (billionths of a metre) in size.

In addition, AIM can see structures that were thought only to occur in "normal" clouds in the lowest few kilometres in the atmosphere. The team believes these features form and break up as a result of pressure waves coming up from below.

"There are rings in the clouds that appear quite frequently," Professor Gary Thomas, an AIM co-investigator from the University of Colorado.

"They're extremely variable. In just a few minutes, these holes are gone and others can appear. And some of these rings are huge - 300-400 miles across."

Noctilucent clouds need cold temperatures, the presence of water vapour, and small dust particles around which the water can condense and freeze out to create ice crystals.

Something must be changing in this "recipe" to alter the clouds' behaviour in recent years. The AIM team is confident its spacecraft will take it closer to understanding the key factors in play.

Jonathan.Amos-INTERNET@bbc.co.uk

SEE ALSO
Mission aims at highest clouds
26 Apr 07 |  Science/Nature
Sky at night takes on a blue hue
05 Jun 06 |  Northern Ireland
Mission to target highest clouds
26 May 06 |  Science/Nature
Spacecraft seek climate clarity
28 Apr 06 |  Science/Nature

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