ionosphere
ionosphere The part of the atmosphere that lies above about 80 km altitude, with the highest concentrations of ions and free electrons. The most intense concentration is at 100–300 km altitude. Long-distance radio communications use waves that are reflected by certain regions of the ionosphere where there are particular concentrations of ions and free electrons. This allows radio waves to be transmitted around the curved surface of the Earth. Communications satellites make it easier to transmit higher-frequency waves (e.g. television transmissions) around the Earth, but reflection from the ionosphere continues to be used for radio transmissions, being cheaper.
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ionosphere
