Thermosphere

The thermosphere is the fourth layer of the Earth's atmosphere. It is 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 X-rays and extreme ultraviolet radiation from the Sun. 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. The absorbed ionizing radiation also creates 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.

It is also characterized by large variability, in response to changes in solar ultraviolet radiation and solar-driven geomagnetic activity.  

 References 
Category:Layers of Earth's atmosphere