thermosphere When the various layers of the atmosphere are described in terms of their temperature structure, the thermosphere is the uppermost layer. It extends from about 80 km to the top of the atmosphere. Thermos is Greek for ‘hot’, and this is the hottest part of the atmosphere. At 80 km the mesopause is the coldest part of the atmosphere, but as altitude increases temperature also increases. In the absence of a heat source temperature will decrease with height. The heat source in the thermosphere is radiation and energetic particles in the solar wind. The Sun emits not only sunlight but also ultraviolet light, X-rays, and a stream of charged particles, electrons, and ions. These bombard the upper part of the atmosphere and if they are sufficiently energetic they can disrupt the atomic and molecular structure of the atmospheric gases.

An individual molecule may respond in several different ways when it absorbs radiation or collides with an incoming particle. It may absorb some of the energy in a collision so that the incoming particle moves off with less energy. It may absorb radiation and change its internal energy states. It may absorb so much energy that the forces binding the molecule together can be overcome and the molecule can split into its component atoms. All these processes can and do happen in the thermosphere and all of them contribute to an increase in the energy of the atmospheric gases which is equivalent to an increase in temperature. Because these processes can result in ionization, this region of the atmosphere is also known as the ionosphere.

At lower levels in the atmosphere the predominant gases are nitrogen (78 per cent by volume) and oxygen (21 per cent by volume). (These figures neglect water vapour, which varies between 4 per cent and negligible amounts.) Both oxygen and nitrogen exist in molecular form, that is, each molecule is formed from two atoms, N₂ and O₂. In the thermosphere both gases suffer changes through collisions and absorption of radiation. Oxygen is readily split into its atomic state so that each molecule becomes two oxygen atoms which have more energy than the complete molecule. The molecular bonds of nitrogen are stronger than those of oxygen, and it is therefore not separated, but it can have electrons removed to become ionized. In the lower parts of the thermosphere, molecular nitrogen is still the most common constituent, but atomic oxygen (O) is more common than molecular oxygen (O₂). At altitudes greater than 180 km, atomic oxygen becomes more common even than molecular nitrogen. This is partly because all the molecular oxygen is now in atomic form. It is also due to the fact that molecular nitrogen is much heavier than atomic oxygen and gravity is stratifying the heavier species at lower levels, leaving only lighter species at higher levels. Gravity does not have this effect below the thermosphere, because turbulence is sufficient to ensure homogeneous mixing of all the gases. However, at 100 km altitude the distance that an atom or molecule will move before colliding with another is, on average, 0.1 m. This is a hundred million times farther than at the Earth's surface. At 180 km this distance has increased to 100 m between collisions.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co.
McIlveen, J. F. R. (1986) Basic meteorology. Van Nostrand Reinhold, New York.

Thermosphere

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

thermosphere An outer layer of the Earth's atmosphere, which includes the ionosphere, extending from an altitude of 85 km (above the mesopause) to the lower level of the exosphere at 500 km. Temperatures increase with altitude in the thermosphere, reaching 1500 °C at 500 km above the Earth's surface. Phenomena which occur in the thermosphere include meteors and the aurora.

ther·mo·sphere / ˈ[unvoicedth]ərmōˌsfir/ • n. the region of the atmosphere above the mesosphere and below the height at which the atmosphere ceases to have the properties of a continuous medium. The thermosphere is characterized throughout by an increase in temperature with height.

thermosphere The upper zone of the atmosphere, above about 80km, where solar radiation of the shortest wavelengths is absorbed. In this zone, which includes the ionosphere, temperature increases with height, but because of the very low atmospheric density there, the heat capacity is minute. See also ATMOSPHERIC STRUCTURE.

thermosphere The upper zone of the atmosphere, above about 80 km, where solar radiation of the shortest wavelengths is absorbed. In this zone, which includes the ionosphere, temperature increases with height, but because of the very low atmospheric density there, the heat capacity is minute.

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