OT, but since you’re talking about diffraction, here is an interesting question 
This is a good question! I like this explanation better: (Ionosphere - Wikipedia)
Since the ionosphere is a plasma, it can be shown that the refractive index is less than unity. Hence, the electromagnetic “ray” is bent away from the normal rather than toward the normal as would be indicated when the refractive index is greater than unity.
In short: it is a refraction but to the same direction as an optical reflection.