Why is the sun's outer atmosphere so staggeringly hot? AI could help us find out

It should not be a mystery. It's because of density.....and then velocity. The temperature is due to velocity. On the surface and below, the environment is liquid like, and dense. This density does not allow the particles to reach a high velocity, because of collision. So the temp is limited. But up away from the liquid, in the atmosphere.....is much less dense, and the particles, can be accelerated to a much higher velocity before collision. Thus, the particles are hotter.

Simple mechanics.