The thermosphere is a layer high in the atmosphere known for extremely high temperatures and extremely low density. From about 50 miles above the surface of Earth upwards to 300 to 600 miles, what few air molecules exist are being bombarded by high-energy solar radiation, which keeps these molecules highly agitated and so, by definition, hot.
Temperatures range between 1,000 and 4,500 degrees Fahrenheit, depending on solar activity.
However, this air would not feel warm to us because it is so thin as to be almost a vacuum. In fact, throughout most of the thermosphere, sound waves cannot be transmitted because of the lack of a medium. It is within the thermosphere that the aurora borealis sometimes glow. The thermosphere is the outermost layer of the atmosphere. Beyond it there is space.