Convexity of the atmolayer
But the difference between the refractive indices of Vacuum and Atmosphere (what determines the amount of refraction that occurs) is not near enough to cause that much bending of light that would be needed to account for what is observed.
It is one thing to propose a mechanism, and another to propose a mechanism that is of sufficient magnitude to account for the observed effect.
Using a RET analogy: if I were to say that Glow Worms have mass and mass produces gravity, then claim that Satellites were kept in orbit because of the mass of glow worms, then, although I am 100% correct (according to RET) that glow worms have mass and therefore gravity, it is of an insufficient magnitude to account for the orbit of Satellites.
All known laws of refraction, and all experiments and all observations that have ever been done with light state that the Refractive index between Vacuum and Atmosphere is completely insufficient to account for the effects you are ascribing to it. So there is no evidence that what you are saying is true and plenty of evidence that says it is false.