NLCs are clouds (actually, they are referred to as cloud-like atmospheric phenomena) that are found in the Earth’s upper atmosphere in the vicinity of 50 to 85 kilometers above the Earth’s surface. They are also called polar mesospheric clouds (PMCs) because they are generally found in the mesosphere and at latitudes from about 50 to 60 degrees north and south of the equator.

The clouds are visible only when the Sun’s rays illuminate them from below the horizon while the ground and lower atmospheric layers are in the Earth’s shadow. That is, they are only seen just before sunrise or after sunset (twilight hours) when sunlight scattered in the upper atmosphere illuminates the lower atmosphere and the surface of the Earth.

Meteorologists do not know very much about noctilucent clouds. First observed in 1885, meteorologists think the clouds are made of frozen ice crystals that form in the cold polar region of the mesosphere. In recent years, they appear to become more frequent, more bright, and to be moving to lower latitudes.

Not much is known about the mesosphere, too. This lack of knowledge of this layer is mostly because it is an atmospheric layer that is too high for aircraft to fly and too low for most spacecraft. Generally, the mesosphere is the layer of atmosphere between the stratosphere and the thermosphere (with the top layer of atmosphere called the exosphere).

Scientists, for the most part, are perplexed as to why these clouds are even present at such lofty altitudes and at such low air pressures. For these reasons and more, the AIM mission was developed.

The AIM mission is to measure such things as water vapor and other atmospheric gases, dust particles, and temperature of these clouds and the amount of dust coming in from outer space. Scientists want to know how these clouds came about and why they seem to change over time.

The AIM satellite, launched by a Pegasus XL launch vehicle from Vandenberg Air Force Base at 1:26:49 p.m. Pacific Daylight Time (PDT) on April 25, will orbit about the Earth at a polar orbit with a height of about 600 kilometers. It will make measurements using the instruments called the Cloud Imaging and Particle Size (CIPS), Cosmic Dust Experiment (CDE), and Solar Occultation for Ice Experiment (SOFIE). Images will be made with four cameras positioned at different angles in order to get unique views of the NLCs.

The results discovered on this mission will provide the foundation for further study of long-term variability in the mesospheric climate.

The Web home of AIM is: http://aim.hamptonu.edu/.

{moscomment}