Mar. 29, 2001 Washington - Scientists have for the first time obtained measurements of upper atmosphere temperatures, iron densities, and mesospheric clouds over the North and South Poles. As reported in the April 1 issue of Geophysical Research Letters, published by the American Geophysical Union, they used a sensitive lidar (radar-like laser) system, which was first deployed over Okinawa, Japan, to observe meteor trails during the 1998 Leonid meteor shower. University of Illinois researchers have now used it to probe temperatures in the upper atmosphere over both geographic poles.
"Measuring temperature profiles over the poles is essential for validating global circulation models and for providing a baseline for assessing the impact of global warming over the coming decades," said team leader Chester Gardner, a professor of electrical and computer engineering. "Until now, we were limited to measurements taken with balloon-borne sensors to altitudes of less than 20 miles [32 kilometers]."
In collaboration with scientists at The Aerospace Corporation and the National Center for Atmospheric Research (NCAR), Gardner and his UI colleagues -- Professor George Papen, research scientist Xinzhao Chu, and graduate student Weilin Pan -- developed a more robust lidar system for measuring temperature profiles from the middle of the stratosphere (about 32 kilometers or 20 miles up) to the lower thermosphere at the edge of space (about 110 kilometers or 70 miles above Earth). The system uses two powerful lasers operating in the near ultraviolet region of the spectrum and two telescopes to detect the laser pulses reflected from the atmosphere.
The researchers use two techniques for determining temperature. For altitudes up to 80 kilometers [50 miles], they measure the amount of laser light reflected from air molecules to derive the temperature profile. For higher altitudes, they measure the scattering of the laser beams from iron atoms deposited in the upper atmosphere by vaporized meteors.
In June 1999, the scientists flew the lidar system over the North Pole aboard an NCAR research plane to obtain temperature and iron density measurements during the Arctic Mesopause Temperature Study. Six months later, they took the instrument to the Amundsen-Scott South Pole Station where it is now measuring the atmospheric temperature structure throughout the year. The National Science Foundation provided funding for the two measurement campaigns.
"Temperature profiles obtained in the thermosphere over the North Pole on June 21, 1999, and in the mesopause region over the South Pole on January 27, 2000, agreed closely with model predictions," Gardner said. "Significant departures from the model were observed during the austral fall, however. On May 8, 2000, for example, the lower mesosphere was about 20 degrees [Celsius; 36 degrees Fahrenheit] warmer and the upper mesosphere was about 20 degrees [Celsius; 36 degrees Fahrenheit] cooler than predicted." The mesosphere extends from the upper limit of the stratosphere, around 80 kilometers [50 miles] above sea level to the mesopause, its upper boundary. The thermosphere begins beyond the mesopause.
Gardner and his colleagues also measured the heights of polar mesospheric clouds that formed over each of the poles during mid-summer. Unlike the lower atmosphere, the upper atmosphere is colder during summer than in winter. Polar mesospheric clouds form over the summertime polar caps when temperatures fall below minus 125 degrees Celsius [-193 degrees Fahrenheit].
These clouds are the highest on Earth, forming at an altitude of about 84 kilometers [52 miles]. Their brightness and geographic extent have been increasing during the past four decades. Scientists believe that these changes may be related to increasing levels of atmospheric carbon dioxide and methane, which in the upper atmosphere lead to cooler temperatures and increasing levels of water vapor.
Surprisingly, the altitudes of the polar mesospheric clouds over the South Pole were consistently two to three kilometers [one to two miles] higher than those over the North Pole. "Higher polar mesospheric clouds may be an indication of stronger upwelling in the summer mesosphere over Antarctica compared with the North polar cap," Gardner said. "Stronger upwelling would result in a cooler mesopause region."
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