What type of radio wave follows a path from the transmitter to the ionosphere and back to Earth?

A radio wave that travels upward from the transmitter, interacts with the ionosphere, and is bent (refracted) back down to Earth is called a sky wave. This phenomenon is the primary mechanism that allows amateur radio operators to communicate over thousands of kilometers on the High Frequency (HF) bands. Direct waves, by contrast, travel in a straight line-of-sight path between antennas and do not interact with the ionosphere. Scattered waves result when signals bounce randomly off uneven atmospheric layers or objects, which is not the reliable, continuous path described in the question.