Background (Encyclopedia of Global Warming)
The lower thermosphere is the region of the atmosphere above the mesopause-lower thermosphere boundary (MLT, about 90 kilometers above the Earth’s surface). It extends to an altitude of about 550 kilometers. Atmospheric density decreases with altitude, as the numbers of atoms and molecules decrease and temperatures rise. The lower thermosphere is sometimes called the ionosphere, because the atmosphere there is highly charged by energetic solar photons.
Solar radiation drives photochemical reactions and motions in Earth’s atmosphere. Careful monitoring of total solar output has revealed that it can vary by as much as 0.1 percent. This magnitude of variability would have little direct effect on Earth’s surface temperatures. However, bursts of activity on the Sun, such as flares and coronal mass ejections, release large amounts of extreme ultraviolet radiation (EUV), soft solar X rays, and radio waves that cause ionization to strengthen in Earth’s thermosphere, creating visible auroras over the polar regions. Above altitudes of 100 kilometers, ambient temperatures may be 1,000° Celsius higher at solar maximum than at solar minimum.
The supposition that variations in solar activity could influence Earth’s weather and climate began several centuries ago. More than a millennium of solar observations have demonstrated that the appearance of sunspots follows irregular eleven-year and twenty-two-year cycles and that the...
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Space Weather and the Thermosphere (Encyclopedia of Global Warming)
When the Sun is active, the Earth’s magnetic field (which extends beyond the thermosphere to the magnetosphere) is strengthened, lessening the number of galactic cosmic rays entering Earth’s atmosphere. The solar magnetic flux affecting Earth has doubled in the last one hundred years. Protons in the inner Van Allen belt (700 kilometers to 10,000 kilometers above Earth) having energy above 50 million electron volts are assumed to arise from the decay of neutrons produced by galactic cosmic-ray collisions in the high atmosphere.
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Indirect Human Modifications of the Thermosphere (Encyclopedia of Global Warming)
Carbon dioxide (CO2) concentration continues to increase in Earth’s atmosphere, and increased CO2 reaching the lower thermosphere has caused cooling. As the thermosphere cools, atmospheric gases settle, causing the air density at an altitude of 400 kilometers to decrease by about 2 percent. This lowered density has decreased drag on satellites, extending their useful lifetimes. Atmospheric drag causes unpowered objects below altitudes of 480 kilometers to fall back to Earth within months. Because drag above this altitude is lower, more debris remains in orbit at that height for extended periods of time.
Most satellites orbit Earth in the lower thermosphere, from communications satellites to the Space Shuttle and International Space Station (ISS) to weather surveillance satellites. They circle the globe in low Earth orbit (LEO), from about 160 kilometers to 1,600 kilometers above the Earth’s surface.
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Pollution of the Thermosphere (Encyclopedia of Global Warming)
Low Earth orbit altitudes are monitored for space junk by ground-based radar systems that track objects larger than 5 centimeters. The lower thermosphere is occupied by hundreds of thousands of old satellite parts; on March 11, 2009, the Space Station was temporarily evacuated because of a near miss with a piece of junk about 12 centimeters in diameter.
On February 10, 2009, two LEO satellites (one defunct and one operational) collided over Siberia at an altitude of 750 kilometers, producing over 160,000 pieces of debris. Proliferation of debris by collisions of orbiting space junk is referred to as the Kessler syndrome: As more objects occupy the lower thermosphere, the chance of collision increases, and each collision creates even more debris, further increasing the chance of collisions.
Detonation of a nuclear weapon at an altitude of 420 kilometers above Johnson Island on July 8, 1962, generated an electromagnetic pulse (EMP) that disrupted electrical systems in Hawaii, 1,290 kilometers away. It also damaged most orbiting satellites. Detonation of a few nuclear weapons in the thermosphere at the start of a major war might cripple ground-based retaliation capabilities by destroying conventional communications that use integrated circuits, leaving intact only “hardened,” or specially shielded, communications systems. (This was the reason for proposed construction of the extremely low frequency grid...
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Context (Encyclopedia of Global Warming)
The thermosphere has undergone great change because of anthropogenic activity in the last fifty years. Increased anthropogenic CO2 emissions have decreased drag at LEO, extending satellite lifetimes. Debris from satellites has rendered LEO more dangerous to piloted space missions and poses a steadily increasing hazard to functioning satellites. During the last century, the Sun has been more active, which may have caused solar influences on the Earth’s atmosphere to increase. During active solar periods in the past, Earth’s climate has apparently warmed, and when the Sun was less active, global cooling occurred. The mechanisms by which the small increases in total solar output might cause climate change on Earth are not known.
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Further Reading (Encyclopedia of Global Warming)
Arnold, Neil. “Solar Variability, Coupling Between Atmospheric Layers and Climate Change.” Philosophical Transactions of the Royal Society of London Annual 360 (2002): 2787-2804. Excellent technical survey of the relationship between solar activity and the atmosphere, including mechanisms that cause solar activity to influence Earth’s climate.
Barth, Charles A., Scott M. Bailey, and Stanley Soloman. “Solar-Terrestrial Coupling: Soft Solar X-Rays and Thermospheric Nitric Oxide.” Geophysical Research Letters 26, no. 9 (1999): 1251-1254. Report on satellite observations that soft solar X rays determine nitric oxide concentrations at an altitude of 110 kilometers over Earth’s tropics during periods of low solar activity.
Glickman, Todd S., ed. Glossary of Meteorology. 2d ed. Boston: American Meteorological Society, 2000. This standard reference book contains definitions of meteorological terms.
Hitchens, Theresa. “Space Wars: Coming to the Sky Near You?” Scientific American 238, no. 3 (March, 2008): 78-85. Describes the consequences of militarizing the thermosphere and proliferating space junk.
Meinel, Aden, and Marjorie Meinel. Sunsets, Twilights, and Evening Skies. New York: Cambridge University Press, 1983. Discusses the consequences of detonating nuclear weapons in the thermosphere.
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Thermosphere (World of Earth Science)
Based on the vertical temperature profile in the atmosphere, the thermosphere is the highest layer, located above the mesosphere. While in the troposphere and the mesosphere, the temperature decreases with altitude. In the stratosphere and thermosphere the temperature increases with height (called temperature inversion). It is separated from the mesosphere by the mesopause, in which the temperature does not change much vertically. Above the thermosphere, the upper limit of the atmosphere, the exosphere can be found blending into space. The upper part of the mesosphere and a big part of the thermosphere overlap with the ionosphere, which is a region defined on the basis of electric properties. The thermosphere and the exosphere together form the upper atmosphere.
Among the four atmospheric temperature-defined layers, the thermosphere is located highest above Earth's surface, beginning at about 57 mi (90 km) above Earth, and reaching into about 300 mi (500 km) height. The name of this layer, thermosphere, originates from the Greek thermo, meaning heat, because in this layer the temperature increases with altitude reaching temperatures higher than 1830°F (1000°C). In the thermosphere, oxygen molecules absorb the energy from the Sun's rays, which results in the warming of the air. Because there are relatively few molecules and atoms in the thermosphere, even absorbing small amounts of solar energy can significantly increase the air temperature, making the thermosphere the hottest layer in the atmosphere. Above 124 mi (200 km), the temperature becomes independent of altitude.
Because the thermosphere and exosphere belong to the upper atmosphere, the density of the air in addition to the atmospheric pressure is greatly reduced when compared to the atmosphere at Earth's surface. At these high altitudes, the atmospheric gases tend to sort into layers according to their molecular mass, and chemical reactions happen much faster here than near the surface of the earth.
See also Atmospheric composition and structure