Thermosphere

thermosphere [′thər·mə‚sfir]

(meteorology)

The atmospheric shell extending from the top of the mesosphere to outer space; it is a region of more or less steadily increasing temperature with height, starting at 40 to 50 miles (70 to 80 kilometers); the thermosphere includes, therefore, the exosphere and most or all of the ionosphere.

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Thermosphere 

the layer of the upper atmosphere located between the mesopause—that is, the upper boundary of the mesosphere—and the base of the exosphere. On the average, the thermosphere extends from a height of about 80 km to a height of about 500 km; these limits vary by ±10–20 percent. In English, the term “thermosphere” is often used in a broader sense to mean the region of the atmosphere extending from the mesopause to outer space. In the present article, the term is used in the narrower sense.

The thermosphere is characterized by a positive temperature gradient, which is equal to zero at the mesopause, has a maximum value between 100 and 200 km, and again becomes equal to zero near the base of the exosphere, where the atmosphere becomes practically isothermal. From the mesopause to the exosphere, the temperature increases approximately from 200°K to 1000°–2000°K. Temperature variations are especially large at the base of the exosphere.

The density of the thermosphere varies on the average from 1.8 × 10^–8g/cm³ at a height of about 80 km to 1.8 × 10^–15g/cm³ at a height of about 500 km. The relative content of the atmospheric components at the mesopause is similar to that at ground level, but the amount of oxygen in the atomic state increases with increasing height. At a level of about 120 km, the separation of gases by diffusion begins. Above the 200–300-km level atomic oxygen, which is lighter than molecular oxygen and nitrogen, becomes predominant; above 500 km, there are significant relative concentrations of even lighter elements, namely, hydrogen and helium. Some of the atoms and molecules in the thermosphere are ionized and are concentrated in several layers (seeIONOSPHERE).

All the characteristics of the thermosphere are subject to rather sizable variations, depending on geographic position, solar activity, season of year, and time of day. The temperature and dynamical conditions of the thermosphere are governed by the energy it absorbs. This energy may originate from nonterrestrial sources or the underlying troposphere.

The major sources of thermospheric energy are: hard solar electromagnetic radiation, which dissociates and ionizes the atmosphere; energetic charged particles (protons and electrons) that penetrate the high-latitude regions of the atmosphere during auroras; atmospheric molecules dissociated into atoms; gravity waves, which may originate both in the troposphere and in the upper atmosphere in the auroral region; and energy dissipation in the thermospheric circulation. The dominant constituents of the thermosphere are nitrogen and oxygen molecules and oxygen atoms, none of which can emit infrared radiation in large amounts. Thus, as a result of insufficient emissivity, the thermosphere is strongly heated, especially at great heights. Under these conditions, heat can be removed only by conduction to the mesopause as a consequence of the positive temperature gradient. A large number of complex molecules, such as carbon dioxide, water, and ozone, are present at the mesopause; these molecules are good emitters of infrared radiation and thus ensure that heat accumulated above the mesopause is removed from the atmosphere.

The thermosphere exerts a drag on artificial satellites, and the behavior of the ionosphere depends substantially on the state of the thermosphere.

REFERENCES

Okolozemnoe kosmicheskoe prostranstvo. Moscow, 1966. (Translated from English.)
Fizika verkhnei atmosfery Zemli. Edited by G. S. Ivanov-Kholodnyi. Leningrad, 1971. (Translated from English.)
Krasovskii, V. I. Shtili i shtormy v verkhnei atmosfere. Moscow, 1971.

V. I. KRASOVSKII