The name thermosphere itself suggests that this layer of atmosphere is hot, that’s why they’ve named this particular layer as Thermosphere. The layer absorbs UV radiation and X-ray to its fullest, which increases the temperature rapidly. The high-energy radiation from the sun is absorbed by the atoms of nitrogen and oxygen. The radiation is converted to heat, energy, and temperature. The temperature can range more than 2000-degrees celsius. Hence, these are the reasons for the thermosphere to be so hot.