The mesosphere is the region of the atmosphere extending from the top of the stratopause (about 50 kilometers high) to the bottom of the mesopauselower thermosphere boundary (MLT), about 100 kilometers high. It is the coldest region of the atmosphere.

The warmest temperatures in the mesosphere are found just above the stratopause, where air temperatures may be as high as ?5œ Celsius. Mesospheric temperatures decrease with increasing altitude, with the lowest temperatures, around 125œ Celsius, occurring during summer at the mesopause (Arnold, 2002).

The natural source of water in the mesosphere is oxidation of methane. Atmospheric concentrations of the greenhouse gas (GHG) methane have increased dramatically since the beginning of the Industrial Revolution and may have influenced the appearance of noctilucent clouds. Water vapor emitted by rockets and the Space Shuttle has been observed to form noctilucent clouds. Exhaust from one shuttle mission may increase the appearance of noctilucent clouds by as much as 20 percent. Carbon dioxide (CO2) in the mesosphere releases heat to space. Solar proton events have been observed to cool the lower mesosphere by causing photochemical reactions leading to ozone depletion, causing an estimated temperature drop of up to 3œ Celsius. Gravity waves are the dominant form of motion in the mesosphere, with wavelengths of around 10 kilometers. Wind speeds on the order of 100 meters per second may occur in the MLT. Gravity wave flux is lowest at solar maximum. Direction of atmospheric transport is from the summer hemisphere to the winter hemisphere.

In the mesosphere, clouds can form only at the coldest temperatures, within about thirty days of the summer solstice at latitudes above 50œ north and below 50œ south. Because they form at 82 kilometers altitude, the thin filamentous noctilucent clouds (sometimes called polar mesospheric clouds, or PMCs) are illuminated only when the Sun is between 6œ and 16œ below the horizon.

There has been speculation that noctilucent clouds are a recent phenomenon resulting from anthropogenic activity, as they were not reported prior to 1885, when scientists began studying the colorful twilight displays that followed the eruption of Krakatoa. Soon after the discovery of noctilucent clouds, speculations began that the particles composing the clouds were of extraterrestrial origin. In 1962, rockets launched to investigate noctilucent clouds determined that the cloud particles had nuclei of iron or nickel dust of extraterrestrial origin, which were coated by water ice. One of the most surprising aspects of noctilucent clouds is their strong radar reflectivity. After some study, an explanation was advanced in 2008 that molecules of sodium and molecules of iron form a thin metallic film over the tiny ice crystals. This film reflects radar waves in a special, amplified way that makes the clouds more prominent on radar than if they were a cloud of disordered metal dust. Noctilucent clouds are postulated to remove about 80 percent of the sodium and iron from their environment in the mesosphere.

Water is formed in the mesosphere as a result of the oxidation of methane gas, rather than arising from the Earth's surface. Because methane is released as a by-product of incomplete combustion, as well as through coal-mining activity and anaerobic production from animals, swamps, and rice paddies, its concentration has been rising steadily as a result of anthropogenic activity. The concentration of methane in the atmosphere has increased 150 percent since the industrial age began, around 1750 (Bellan, 2008).

After rockets are launched through the mesosphere, their exhaust trails may form artificial noctilucent clouds. These noctilucent trails can form in the mesosphere in temperate latitudes, but ground observations of them are made only when the Sun is between 6œ and 16œ below the horizon. After the launch of the STS-107 shuttle mission in January, 2003, it was determined that vaporized iron and water from its exhaust traveled to Antarctica at 110 kilometers altitude in about two days. This exhaust plume went on to form noctilucent clouds in the southern polar summer. Data from this event indicate that a single Space Shuttle's exhaust might cause a 10 percent to 20 percent increase in the appearance of seasonal noctilucent clouds.

References

1. Arnold, Neil. "Solar Variability, Coupling Between Atmospheric Layers, and Climate Change." Philosophical Transactions of the Royal Society of London A 360, no. 1801 (December, 2002): 2787-2804.

2. Bellan, Paul M. "Ice Iron/Sodium Film as Cause for High Noctilucent Cloud Radar Reflectivity." Journal of Geophysical Research D 113 (2008): 16,215- 16,218.

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