The upper atmospheric layer above the mesopause (c.80 km/50 mi, separating the mesosphere from the thermosphere) in which atmospheric densities are very low. The lower part is composed mainly of nitrogen (N2) and oxygen in molecular (O2) and atomic (O) forms, whereas above 200 km/125 mi atomic oxygen predominates over N2 and N. Temperatures increase with altitude because of the absorption of ultraviolet radiation by atomic oxygen. The term ionosphere is also used for the atmosphere above 80 km/50 mi.
The thermosphere is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere.
The thermosphere, named from the Greek θερμός (thermos) for heat, begins about 85 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. 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. Even though the temperature is so high, one will not feel warm in the thermosphere.
The MIR and International space stations have stable orbits within the upper part of the thermosphere, between 320 and 380 kilometers.
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