In addition to being subject to the tug of planetary gravity, microscopic particles orbiting the sun are pushed outward by the pressure of sunlight, dragged inward by their
own radiative emissions, and worn down by collisions with other particles.
In this explanation what is changing is the altitude at which emission occurs, and at higher levels it is colder, so this level needs to warm up to maintain the radiative balance; what would happen once the altitude of
radiative emission reaches above the tropopause?
The power of a laser depends on the gain of the material it is made of, and this gain is proportional to
the radiative emission rate.
You can take a look at the details of
the radiative emission and absorption, but that is about it.
Notice that we have NOT invoked a specific model for
the radiative emissions (yet).