Region of the Earth's atmosphere in which the number of ions, or electrically charged particles, is large enough to affect the propagation of radio waves.
The ionosphere begins at an altitude of about 30 mi (50 km) but is most distinct above about 50 mi (80 km). The ionization is caused mainly by solar radiation at X-ray and ultraviolet wavelengths. The ionosphere is responsible for the long-distance propagation, by reflection, of radio signals in the shortwave and broadcast bands.