<P> The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere and below the exosphere . Within this layer of the atmosphere, ultraviolet radiation causes photoionization / photodissociation of molecules, creating ions in the ionosphere . Taking its name from the Greek θερμός (pronounced thermos) meaning heat, the thermosphere begins about 85 kilometres (53 mi) 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 . Temperatures are highly dependent on solar activity, and can rise to 2,000 ° C (3,630 ° F). Radiation causes the atmosphere particles in this layer to become electrically charged (see ionosphere), enabling radio waves to be refracted and thus be received beyond the horizon . In the exosphere, beginning at 500 to 1,000 kilometres (310 to 620 mi) above the Earth's surface, the atmosphere turns into space, although by the criteria set for the definition of the Kármán line, the thermosphere itself is part of space . </P> <P> The highly diluted gas in this layer can reach 2,500 ° C (4,530 ° F) during the day . Even though the temperature is so high, one would not feel warm in the thermosphere, because it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat . A normal thermometer might indicate significantly below 0 ° C (32 ° F), at least at night, because the energy lost by thermal radiation would exceed the energy acquired from the atmospheric gas by direct contact . In the anacoustic zone above 160 kilometres (99 mi), the density is so low that molecular interactions are too infrequent to permit the transmission of sound . </P>

Why is thermosphere the hottest layer of the atmosphere