Why is the highest temperature of all atmosphere layers?

The highest temperature in the Earth's atmosphere occurs in the thermosphere, primarily due to the absorption of high-energy solar radiation by the sparse gas molecules present at this altitude. As these molecules absorb ultraviolet and X-ray radiation, they gain kinetic energy, leading to significantly higher temperatures, which can exceed 2,500 degrees Celsius (4,500 degrees Fahrenheit). However, despite these high temperatures, the thermosphere would not feel hot to a human because of the extremely low density of air at that altitude.