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Caltech Scientist Proposes Explanation for Puzzling Property of Night-Shining Clouds at the Edge of Space
An explanation for a strange property of noctilucent clouds--thin, wispy clouds hovering at the edge of space at 85 km altitude--has been proposed by an experimental plasma physicist at the California Institute of Technology (Caltech), possibly laying to rest a decades-long mystery.

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Last month, astronauts on board the International Space Station (ISS) witnessed a beautiful display of noctilucent or "night-shining" clouds. The station was located about 340 km over western Mongolia on July 22nd when the crew snapped this picture:

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Above: Noctilucent clouds photographed by the crew of the ISS:

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Noctilucent clouds, also known as polar mesospheric clouds, are bright cloudlike atmospheric phenomena visible in a deep twilight. The name means roughly "night shining" in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.

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NASA's AIM satellite has provided the first global-scale, full-season view of iridescent polar clouds that form 50 miles above Earth's surface.
 The Aeronomy of Ice in the Mesosphere (AIM) mission is the first satellite dedicated to the study of these noctilucent or "night-shining" clouds. They are called "night shining" clouds by observers on the ground because their high altitude allows them to continue reflecting sunlight after the sun has set below the horizon. AIM has provided the first global-scale view of the clouds over the entire 2007 Northern Hemisphere season with an unprecedented horizontal resolution of 3 miles by 3 miles.
Very little is known about these 'clouds at the edge of space', also called Polar Mesospheric Clouds. How do they form over the summer poles, why are they being seen at lower latitudes than ever before, and why have they been growing brighter and more frequent? During its mission lifetime, AIM will observe a total of two complete polar mesospheric cloud seasons in each polar region, documenting for the first time the entire complex life cycle of Polar Mesospheric Clouds.

"The AIM mission has changed our view of Polar Mesospheric Clouds and their surroundings after only one season of observations. The measurements show the brightest clouds ever observed with more variability and structure than expected, signifying extreme sensitivity to the environment in which the clouds form. They also show that the clouds exist over a broader range in height than was believed to be the case before AIM was launched. The unprecedented sensitivity has revealed for the first time the presence of very small ice particles believed to be responsible for the mysterious radar phenomenon known as "Polar Mesospheric Summertime Echoes" - AIM Principal Investigator James Russell III of Hampton University, Hampton, Va.

The bright "night-shining" clouds were seen by the spacecraft's instruments regularly, first appearing on May 25th and lasting until August 25th.
 The AIM satellite reported daily observations of the clouds at all longitudes and over a broad latitude range extending from 60 degrees North to 85 degrees North. The AIM satellite is currently making the first global observations of the Southern Hemisphere cloud season. The clouds consist of ice crystals formed when water vapour  condenses onto dust particles in these coldest regions of our planet, at temperatures that may dip to minus 210 to minus 235 degrees Fahrenheit.

New results from AIM's first Northern Hemisphere season observations show:

1. The most detailed picture of the clouds ever collected showing that they appear every day, they are widespread, and they are highly variable on hourly to daily time scales.

2. That Polar Mesospheric Cloud brightness varies over horizontal  scales of about two miles; and over small regions, clouds measured by AIM are ten-fold brighter than measured by previous space-based instruments.

 3. The unexpected result that mesospheric ice occurs in one continuous  layer extending from below the main peak at 51 miles up to around 55 miles.

 4. Observations of a previously suspected, but never before seen, population of very small ice particles believed to be responsible for strong radar echoes from the summertime mesosphere. This was made possible because of the unprecedented sensitivity of the AIM measurements.

5. Polar Mesospheric Cloud structures resolved for the first time that exhibit complex features present in normal tropospheric clouds. This startling similarity suggests that the mesosphere may share some of the same dynamical processes responsible for weather near the surface. If this similarity holds up in further analysis, this opens up an entirely different view of potential mechanisms that can explain why the clouds form and how they vary.


 The new results were produced by David Rusch and the Cloud Imaging and  Particle Size experiment team, University of Colorado, Laboratory for Atmospheric and Space physics; and Larry Gordley and Mark Hervig and the Solar Occultation for Ice Experiment team, Gats, Inc., Newport News, Va.
 AIM is a NASA-funded Small Explorers mission managed by the Explorers Program Office at Goddard Space Flight Center, Greenbelt, Md. The mission is led by the principal investigator from the Centre for Atmospheric Sciences at Hampton University in Va. Instruments were built by the Laboratory for Atmospheric and Space Physics (LASP), University of Colorado, Boulder, and the Space Dynamics Laboratory, Utah State University in Logan. The LASP also manages the AIM mission and is controlling satellite flight operations. Orbital Sciences Corporation, Dulles, Va., designed, manufactured, and tested the AIM spacecraft, and provided the Pegasus launch vehicle.

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This image of noctilucent clouds above Mongolia was acquired on June 10, 2007, by the STS-117 crew using a Kodak 760C digital camera and 180 mm lens.

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Credit NASA

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