The thermosphere extends from the mesosphere (~85 km) to ~600 km. Temperature rises from −90 °C at its base to 1,000–2,000 °C in its upper reaches during solar maxima, though air density is so low that a conventional thermometer would not register that heat.
It is the layer where auroras occur (100–300 km) and where the International Space Station orbits (400 km). It contains the ionosphere, an ionised layer that reflects radio waves and enables long-distance shortwave communications. Solar activity strongly modulates the thermosphere.