Editing Kennelly–Heaviside layer (section)

==History==
Existence of a reflective layer was predicted in 1902 independently and almost simultaneously by the American electrical engineer [[Arthur Edwin Kennelly]] (1861–1939)<ref name=Russell_1925 /> and the British polymath [[Oliver Heaviside]] (1850–1925), as an explanation for the propagation of radio waves beyond the horizon observed by [[Guglielmo Marconi]] in 1901. However, it was not until 1924 that its existence was shown by British scientist [[Edward V. Appleton]],<ref>Appleton, Edward V., and Barnett, M. A. F. (1 December 1925). [https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1925.0149 "On some direct evidence for downward atmospheric reflection of electric rays"], ''Proceedings of the Royal Society of London, Series A, Containing Papers of a Mathematical and Physical Character,'' '''109'''.752 pp. 621–641.</ref> for which he received the 1947 [[Nobel Prize in Physics]].<ref>"[https://www.nature.com/articles/160703c0 Nobel Prizes for 1947: Sir Edward Appleton, G.B.E., K.C.B., F.R.S."], ''Nature'', '''160''', pp. 703–704 (22 November 1947) doi:10.1038/160703c0</ref>

Physicists resisted the idea of the reflecting layer for one very good reason; it would require [[total internal reflection]], which in turn would require that the speed of light in the ionosphere would be greater than in the atmosphere below it. Since the latter speed is essentially the same as the speed of light in vacuum ([[Speed of light|''c'']]), scientists were unwilling to believe the speed in the ionosphere could be higher. Nevertheless, Marconi had received signals in Newfoundland that were broadcast in England, so clearly there must be some mechanism allowing the transmission to reach that far. The paradox was resolved by the discovery that there were two velocities of light, the ''[[phase velocity]]'' and the ''[[group velocity]]''. The phase velocity can in fact be greater than ''c'', but the group velocity, being capable of transmitting information, cannot, by [[special relativity]], be greater than ''c''. The phase velocity for radio waves in the ionosphere is indeed greater than ''c'', and that makes total internal reflection possible, and so the ionosphere can reflect radio waves. The geometric mean of the phase velocity and the group velocity cannot exceed ''c'', so when the phase velocity goes above ''c'', the group velocity must go below it. 

In 1925, Americans [[Gregory Breit]] and [[Merle A. Tuve]] first mapped the Heaviside layer's variations in altitude. The [[ITU]] standard model of absorption and reflection of radio waves by the Heaviside Layer was developed by the British Ionospheric physicist [[Louis Miles Muggleton|Louis Muggleton]]<ref name="ITU">{{Cite journal|author=L.M. Muggleton|year=1975|title=A method of predicting foE at any time and place|journal=ITU Telecommunication Journal|volume=42|pages=413–418|bibcode=1975ITUTJ..42..413M}}</ref> in the 1970s.