What are radio waves called that reflect off the ionosphere before reaching the receiver?

Radio waves that reflect off the ionosphere before reaching the receiver are called sky waves. This phenomenon occurs when the ionosphere, a layer of the Earth's atmosphere that contains ionized particles, reflects certain frequencies of radio waves back to the Earth's surface, allowing them to travel longer distances than would otherwise be possible through direct transmission.

Sky waves are particularly effective for medium and high frequency (MF and HF) bands, as they can bounce off the ionosphere and cover vast geographical areas by enabling communication beyond the visible line-of-sight. This characteristic makes them essential for long-distance radio communications, such as those used in amateur radio and international broadcasting.

Other types of waves, like ground waves, travel along the surface of the Earth and are subject to various forms of attenuation and interference. Direct waves travel in a straight line between the transmitter and receiver without any reflection off the ionosphere, while modulated waves refer to waves that have been altered in their amplitude, frequency, or phase to convey information, rather than specifying their reflective nature.