<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 at about 80 km ( 50 mi ) above sea level . 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 1,700 ° C ( 3,100 ° F ) or more . 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 about 600 km ( 375 mi ) above sea level , 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>
why does the temperature increase in the thermosphere
due to absorption of highly energetic solar radiation