What usually happens to radio waves with frequencies below the LUF?

The Lowest Usable Frequency (LUF) is the threshold where ionospheric absorption becomes too severe for successful communication. This absorption primarily happens in the D region, the lowest layer of the ionosphere, which becomes heavily energized by the sun during daylight hours. When radio waves are transmitted at frequencies below the LUF, they lose all their energy to this lower layer before they can reach the higher F layer to be refracted. As a result, the signal is completely attenuated (absorbed as heat) and will not reach its intended destination.