Ionisation of air molecules in the ionosphere is produced by ultraviolet radiation from the Sun, and to a lesser extent by high-energy particles from the Sun and from cosmic rays.

The large number of free electrons in the ionosphere allows the propagation of electromagnetic waves. Radio signals - a form of electromagnetic radiation - can be "bounced" off the ionosphere allowing radio communication over long distances.

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© Bill Arnett

A region of charged particles in a planet's upper atmosphere; the part of the Earth's atmosphere beginning at an altitude of about 25 miles and extending outward 250 miles or more.