The atmospheric region containing significant concentrations of ions and electrons. Its base is at about 70–80 km and it extends to an indefinite height. In terms of the standard upper-atmospheric nomenclature, the ionosphere is collocated with the thermosphere and the upper mesosphere, while the outer ionosphere also forms part of the magnetosphere. Most of the early knowledge of the ionosphere came from sounding by ground-based radars known as ionosondes, and the sharply defined echoes produced by these instruments led to the definition of ionospheric layers, implying well-defined regions with clear maxima of ionization. More recent investigations by a number of remote and in situ techniques have shown that such sharply bounded layers do not generally exist (with the exception of the so-called sporadic-E layers), and that the layers are better described as regions. The term layer is still frequently used, however. Ionospheric sounding normally shows an E-layer in the 100- to 120-km height range, an F1-layer in the 150- to 190-km range, and an F2-layer above 200 km. The D-region, lying in the upper mesosphere below 100 km, is responsible for daytime absorption of high-frequency radio waves, but does not usually produce an echo on ionosonde recordings. Early suggestions of a C-layer below the D-region, and a G-layer above the F2-layer, have not been generally accepted, and these terms are now obsolete. Over most of the earth, the ionosphere is produced by the action of solar radiation of short wavelength (extreme ultraviolet and x-ray radiation) on the atmospheric constituents. At high magnetic latitudes, energetic particles of solar or auroral origin become important, and even dominant, sources of ionization. See atmospheric shell.