Thermosphere

THE EARTH IS surrounded by a blanket of air, called the atmosphere. The atmosphere is a thin layer of gases that envelope the Earth. The gases are held close to the earth by gravity and the thermal movement of air molecules. Life on Earth is supported by the atmosphere, [Page 966]solar energy, and the magnetic fields. Five layers have been identified in the atmosphere, using thermal characteristics, chemical composition, movement, and density. The atmosphere is divided into the troposphere, the stratosphere, the mésosphère, the thermosphere, and the exosphere. The thermosphere, from the Greek word (thermos) for heat, is the fourth atmospheric layer from Earth, separated from the mésosphère by the mesopause. It begins about 50 mi. (80 km.) above the Earth and is the layer of the atmosphere directly above the mésosphère and below the exosphere. The lower part of the thermosphere, from 50 to 342 mi. (80 to 550 km.) above the Earths surface, contains the ionosphere, which is the region of the atmosphere that is filled with charged particles. Beyond the ionosphere, extending out to perhaps 6,214 mi. (10,000 km.), is the exosphere.

The Earths thermosphere is the layer of the atmosphere that is first exposed to the sun's radiation and so is first heated by the sun; it is the hottest layer of the atmosphere. Within the thermosphere, temperatures rise continually to well beyond 1,832 degrees F (1,000 degrees C). In the thermosphere, ultraviolet radiation causes ionization. At these high altitudes, the residual atmospheric gases sort into strata according to their molecular mass. Thermospheric temperatures increase with altitude as a result of the absorption of highly energetic solar radiation by the small amount of residual oxygen present. Temperatures in the thermosphere are highly dependent on solar activity. Radiation causes the air particles in this layer to become electrically charged, enabling radio waves to bounce off and be received beyond the horizon.

The Northern Lights occur in the thermosphere, which is the fourth atmospheric layer from Earth. It is the first layer of the atmosphere that is exposed to the sun's radiation, so it is the hottest layer of the atmosphere.

The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the sun, causing the layer to warm to high temperatures. Air temperature, however, is a measure of the kinetic energy of air molecules—not of the total energy stored by the air. The air is so thin that a small increase in energy can cause a large increase in temperature. Because the air is so thin within the thermosphere, such temperature values are not comparable to those of the troposphere or stratosphere. Again, because of the thin air in the thermosphere, [Page 967]scientists cannot measure the temperature directly. Instead, they measure the density of the air by how much drag it puts on satellites and then use the density to determine the temperature.

Although the measured temperature is very hot, the thermosphere would actually feel very cold to humans because the total energy of the few air molecules residing there would not be enough to transfer any appreciable heat to our skin. In addition, it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat. A normal thermometer would read significantly below 32 degrees F (0 degrees C). The dynamics of the lower thermosphere are dominated by the atmospheric tide, which is driven, in part, by the very significant diurnal heating.

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