The thermosphere is the fourth layer of the earth's atmosphere which lies directly above the mesosphere (middle layer) and directly below the exosphere (top layer). The name Thermosphere is taken from the Greek word thermos which means heat. Within this layer, ultraviolet radiation causes ionization. That is why it is also called the ionosphere. Shortwave radio waves can bounce off the ionosphere and go to distant parts of the Earth. This layer is very helpful in radio communication.