Why is the thermosphere so hot?

I read that it is because oxygen and nitrogen absorb solar energy. Nitrogen I can understand, but oxygen? What exactly am I missing here?

What is it about oxygen that is confusing you?

Keep in mind that temperature measures an average of an ensemble of lots of molecules interacting with each other and their environment. Thanks to the low pressure in the thermosphere, such interactions between the gas molecules are greatly reduced, and temperature takes a bit of different meaning. For instance, you cannot measure the 'temperature' of the thermosphere with ordinary thermometers.

As elgerak says, in low pressure regimes the "temperature" is measured by the average velocities of the gas molecules around you. It doesn't make much difference which gasses those are. In the sun's chromosphere the gasses are accelerated by magnetic fields and reach temperatures in the millions of degrees. They are so thin, though, that they don't appreciably warm up anything passing through which can control it's radiation budget.

shadron said:

As elgerak says, in low pressure regimes the "temperature" is measured by the average velocities of the gas molecules around you.

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That's also the case in denser regimes. The difference is the "heating" capability of the gas at a certain temperature.

If you put an object (say, a thermometer) into the gas at high densities, the object will quickly assume the same temperature of the gas. In low densities, however, so few gas molecules interact with the object over time that it has time to cool down again by emitting infrared radiation in between collisions with gas molecules. Meaning that the object will assume a lower temperature than the gas around it (as indicated by the velocity or kinetic energy of the gas molecules).

bit_pattern said:

I read that it is because oxygen and nitrogen absorb solar energy. Nitrogen I can understand, but oxygen? What exactly am I missing here?

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Assuming you already know they have about the same specific heat, probably density.

At the surface 1000J/m² would raise the temperature of the air about 1K, in the thermosphere 1000J/m² would raise the temperature about 600K.