Temperature rise in Thermosphere

Question

Generally speaking, temperature decreases as altitude increases from the sea level. But at the ozone layer it increases because ozone absorbs UV radiation. Temperature increases again in Thermosphere. I want to know what is present in Thermosphere (e.g., what kind of gases that absorb radiation are present) and not present, e.g., in Mesosphere.

Answer 1

The gases shift substantially to lower atomic numbers, and you can find many academic references that give modeling references that show this. There is a lot of weather-based variation, particularly in the upper atmosphere densities, but this is extremely well-established science. If you want to know the fraction of Nitrogen at 200 km, you can totally find an estimate, and it will be a good estimate of a real value.

You can follow this stack exchange question, or dive right into calculations. The overwhelming conclusion is that you find Hydrogen dominates as you get to space-level heights. I hesitate to get more specific with the question as it stands, because those specifics would have nothing to do with what you're trying to explain.

The reason that gases don't absorb XUV light in the lower thermosphere region is because those wavelengths are depleted in higher regions of the atmosphere. See:

https://en.wikipedia.org/wiki/Thermosphere#Solar_XUV_radiation

The solar X-ray and extreme ultraviolet radiation (XUV) at wavelengths < 170 nm is almost completely absorbed within the thermosphere. This radiation causes the various ionospheric layers as well as a temperature increase at these heights (Figure 1).

This is a very satisfying explanation for why the temperature profile behaves as it does between the mesopause (the temperature minimum) and space. The heat source follows an attenuation curve as those XUV rays are absorbed, leaving more energy in the higher regions, and less energy in the middle regions. In the very low regions close to Earth's surface, the other parts of the spectrum come into play. So we have two heating sources, and loss to radiation pretty much everywhere. The region in the middle just doesn't receive much from either of those two sources, so it's a lower temperature.

Answer 2

Atoms of nitrogen, oxygen, and helium in the thermosphere are unshielded from incoming UV radiation. Therefore, they absorb the most energetic UV radiation copiously and attain high temperatures. However, because the atoms are so sparse, the high temperature (high kinetic energy) of each individual atom is not able to join with other atoms to form a heat bath, and any object in the thermosphere would be very hot on the sunny side, but very cold on the shady side.