The ionosphere is a part of the upper atmosphere, from about 85 km to 600 km altitude, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth.
Read more about Ionosphere: Geophysics, The Ionospheric Layers, Anomalies To The "ideal" Model, Radio Application, Other Applications, Ionospheres On Other Planets and Titan, History