The thermosphere is the upper layer of the atmosphere, beginning at an altitude of about 80 to 100 kilometres. It is separated from the lower layer, the mesosphere, by the mesopause.
Its lower part includes the ionosphere, an ionised layer between 80 and 600 kilometres. Its upper part is the exosphere, which extends to about 10,000 kilometres.
In the thermosphere, the temperature increases with altitude and rises higher than 1,000 °C. However, the pressure is very low and the temperature provides a measurement of the kinetic energy of air molecules.