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An ionized layer in a planetary atmosphere where free electrons and ions with thermal energies exist under the control of the gravitational and magnetic fields of the planet. The Earth's ionosphere lies between heights of about 50 and 600 kilometres (30 and 350 miles), though the extent varies considerably with time, season and solar activity. It is created by the effect of ultraviolet and X-radiation from the Sun. Four different layers with different characteristics are known as the D, E, F1 and F2 layers in order of increasing height. The D region, between 50 and 90 kilometres, has low electron density. The E and F1 regions, between 90 and 230 kilometres, form the main part of the ionosphere.
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