Thermosphere

The thermosphere is that layer, of the Earth's atmosphere, which is directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. (see also: ionosphere)

The thermosphere, from the Greek thermo for heat, begins about 80 km above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures can rise to 2,000°C. Radiation causes the scattered air particles in this layer to become charged electrically (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000 km 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.