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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. This layer has high temperatures because it absorbs ionizing radiation. Even though it has temperatures of 2,000 °C (3,360 °F), the density of the thermosphere is so low that it doesn't feel hot. Within this layer, ultraviolet radiation causes ionization. That is why it is also called the ionosphere. Shortwave radio waves can bounce off the Kennelly-Heaviside layer (ionosphere) and go to distant parts of the Earth. This layer is very helpful in radio communication.