Ionospheric reflection

Ionospheric reflection: Of electromagnetic waves propagating in the ionosphere, a redirection, i.e., bending--by a complex processing involving reflection and refraction--of the waves back toward the Earth.

Note: The amount of bending depends on the extent of penetration (which is a function of frequency), the angle of incidence, polarization of the wave, and ionospheric conditions, such as the ionization density.

Source: from Federal Standard 1037C