Ionosphere

The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photo ionization of the constituents of the atmosphere by solar ultraviolet radiation at very short wavelengths (less than 1000 angstroms). The ionosphere significantly influences radio wave propagation of frequencies less than about 30 MHz. The existence of the ionosphere was first proposed by Balfour Stewart in 1882 to account for differences in the earth′s magnetic field.

See also: Earth.