The thermosphere is so hot because it absorbs a large amount of the ultraviolet and x-ray radiation coming to Earth from the sun, converting it to heat. Its temperature is extremely variable, based on both the time of day and the sun's activity.
Temperatures within the thermosphere increase at higher levels until it becomes somewhat stable up to the thermopause, the end of the thermosphere and the beginning of the exosphere. In upper levels, the typical temperatures of the thermosphere are usually anywhere between 900 degrees and 3,600 degrees Fahrenheit. This temperature often is about 400 degrees Fahrenheit hotter during the day than at night. It is around 900 degrees Fahrenheit hotter when the sun is very active than when it is inactive.
The thermosphere is considered part of the atmosphere, but it is extremely thin and has different components than the lower atmosphere. Rather than the mix of gases found in the lower atmosphere, different elements tend to be in different regions in the thermosphere. In addition, the gas molecules tend to dissociate into individual atoms, so that the gases in the thermosphere are largely atomic oxygen, atomic nitrogen and helium. These gases are frequently hit by ionizing radiation from the sun, stripping electrons from them.