Thermosphere

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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 ionization. (see also: ionosphere). It is the fourth atmospheric layer from earth.

The thermosphere, named from the Greek θερμός (thermos) for heat, begins about 80 km above the earth. 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 by the small amount of residual oxygen still present. Temperatures are highly dependent on solar activity, and can rise to 2,000°C. Radiation causes the air particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000km above the earth's surface, the atmosphere blends into space. The few particles of gas here can reach 2,500°C (4500°F) during the day. Even though the temperature is so high, one will not feel warm in the thermosphere. A normal thermometer would read significantly below 0°C. This is due to the distance between the present molecules.

The dynamics of the lower thermosphere (below about 120 km) is dominated by atmospheric tide which is driven, in part, by the very significant diurnal heating. The atmospheric tide dissipates above this level since molecular concentrations do not support the coherent motion needed for fluid flow.

The International Space Station has a stable orbit within the upper part of the thermosphere, between 320 and 380 kilometers. The Northern Lights also occur in the upper thermosphere.