That part of the atmosphere containing a sufficient number of ions and electrons to cause appreciable dispersion of radio waves of frequency less than 1 GHz. The lower boundary of the ionosphere is at about 50 km (the upper boundary of the stratosphere) and the upper boundary (to the extent that one can be defined) is at about 2000 km. Four layers can be distinguished in the iono-sphere. The lowest, the D-layer, has an electron density of about 1000 electrons per cc at maximum; it appears only during the day and disappears at night. The maximum occurs at about 80 km. The E-layer extends from about 85 km to 140 km, contains about 100 000 electrons per cc at periods of least solar activity and about 150 000 electrons per cc at times of greatest solar activity. The F-layer extends from about 140 km outwards. The structure of the F-layer is extremely complex (electron density varies from about 500 000 to over 2 000 000 particles per cc). Two regions can easily be distinguished: the F1 region extends from about 140 km to about 200 km; the F2 region extends from 200 km on out. The ionization is caused principally by solar radiation. The D-layer appears to be caused by hard x-rays, the E-layer by soft x-rays, and the F-region by ultraviolet radiation and light.