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The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes photoionization/photodissociation of molecules present. Called from the Greek θερμός (''pronounced thermos'') meaning heat, the thermosphere begins about above the Earth.〔Duxbury & Duxbury. Introduction to the World's Oceans. 5ed. (1997)〕 At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation. Temperatures are highly dependent on solar activity, and can rise to . Radiation causes the atmosphere particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. In the exosphere, beginning at above the Earth's surface, the atmosphere turns into space. The highly diluted gas in this layer can reach during the day. Even though the temperature is so high, one would not feel warm in the thermosphere, because 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 be significantly below , because the energy lost by thermal radiation would exceed the energy acquired from the atmospheric gas by direct contact. In the anacoustic zone above , the density is so low that molecular interactions are too infrequent to permit the transmission of sound. The dynamics of the thermosphere are dominated by atmospheric tides, which are driven by the very significant diurnal heating. Atmospheric waves dissipate above this level because of collisions between the neutral gas and the ionospheric plasma. The International Space Station orbits within the middle of the thermosphere, between (decaying by 2 km/month and raised by periodic reboosts), whereas the Gravity Field and Steady-State Ocean Circulation Explorer satellite at utilized winglets〔http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE/Satellite〕 and an innovative ion engine to maintain a stable orientation and orbit.〔http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE/A_technological_achievement〕 It is convenient to separate the atmospheric regions according to the two temperature minima at about 12 km altitude (the tropopause) and at about 85 km (the mesopause) (Figure 1). The thermosphere (or the upper atmosphere) is the height region above 85 km, while the region between the tropospause and the mesopause is the middle atmosphere (stratosphere and mesosphere) where absorption of solar UV radiation generates the temperature maximum near 45 km altitude and causes the ozone layer. The density of the Earth's atmosphere decreases nearly exponentially with altitude. The total mass of the atmosphere is M = ρA H ≃ 1 kg/cm2 within a column of one square centimeter above the ground (with ρA = 1.29 kg/m3 the atmospheric density on the ground at z = 0 m altitude, and H ≃ 8 km the average atmospheric scale height). 80% of that mass already concentrated within the troposphere. The mass of the thermosphere above about 85 km is only 0.002% of the total mass. Therefore, no significant energetic feedback from the thermosphere to the lower atmospheric regions can be expected. Turbulence causes the air within the lower atmospheric regions below the turbopause at about 110 km to be a mixture of gases that does not change its composition. Its mean molecular weight is 29 g/mol with molecular oxygen (O2) and nitrogen (N2) as the two dominant constituents. Above the turbopause, however, diffusive separation of the various constituents is significant, so that each constituent follows its own barometric height structure with a scale height inversely proportional to its molecular weight. The lighter constituents atomic oxygen (O), helium (He), and hydrogen (H) successively dominate above about 200 km altitude and vary with geographic location, time, and solar activity. The ratio N2/O which is a measure of the electron density at the ionospheric F region is highly affected by these variations.〔Prölss, G.W. and M. K. Bird, "Physics of the Earth's Space Environment", Springer Verlag, Heidelberg, 2010〕 These changes follow from the diffusion of the minor constituents through the major gas component during dynamic processes. ==Energy input== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thermosphere」の詳細全文を読む スポンサード リンク
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