The Heaviside layer, sometimes called the Kennelly–Heaviside layer, named after Arthur E. Kennelly and

Oliver Heaviside Oliver Heaviside FRS (; 18 May 1850 – 3 February 1925) was an English self-taught mathematician and physicist who invented a new technique for solving differential equations (equivalent to the Laplace transform), independently develope ...

, is a layer of

ionised Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

gas occurring roughly between 90km and 150 km (56 and 93 mi) above the ground — one of several layers in the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

. It is also known as the E region. It reflects medium-frequency

radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...

s. Because of this reflective layer, radio waves radiated into the sky can return to Earth beyond the

horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...

. This "

skywave In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature o ...

" or "skip" propagation technique has been used since the 1920s for radio communication at long distances, up to transcontinental distances. Propagation is affected by the time of day. During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. Conversely, on the night (

lee Lee may refer to: Name Given name * Lee (given name), a given name in English Surname * Chinese surnames romanized as Li or Lee: ** Li (surname 李) or Lee (Hanzi ), a common Chinese surname ** Li (surname 利) or Lee (Hanzi ), a Chinese ...

) side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection. The extent of the effect is further influenced by the

season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...

, and the amount of sunspot activity.

History

Existence of a reflective layer was predicted in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861–1939) and the British polymath

(1850–1925), as an explanation for the propagation of radio waves beyond the horizon observed by

Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italian inventor and electrical engineer, known for his creation of a practical radio wave-based wireless telegraph system. This led to Marconi ...

in 1901. However, it was not until 1924 that its existence was shown by British scientist Edward V. Appleton, for which he received the 1947

Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...

.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 Physicists resisted the idea of the reflecting layer for one very good reason; it would require

total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflect ...

, 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 a vacuum ( "''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 In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The law ...

, 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 The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Union ...

standard model of absorption and reflection of radio waves by the Heaviside Layer was developed by the British Ionospheric physicist Louis Muggleton in the 1970s.

Etymology

Around 1910, William Eccles proposed the name "Heaviside Layer" for the radio-wave reflecting layer in the upper atmosphere,Griffiths, Hugh (15 November 2018).
Oliver Heaviside and the Heaviside layer"
''Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences,'' 376: No. 2134. https://doi.org/10.1098/rsta.2017.0459. Retrieved 14 June 2019. and the name has subsequently been widely adopted.Marchant, E. W. (Dec. 1916).
The Heaviside layer"
''Proceedings of the Institute of Radio Engineers, 4,'' No. 6, pp. 511–520. The name Kennelly–Heaviside layer was proposed in 1925 to give credit to the work of Kennelly,McAdie, Alexander (22 May 1925)
The Kennelly-Heaviside Layer"
''Science,'' 61, No 1586, pp. 540 DOI: 10.1126/science.61.1586.540Russell, A. (24 October 1925).
The “Kennelly-Heaviside” Layer"
''Nature'' 116, p. 609. doi:10.1038/116609c0 which predated the proposal by Heaviside by several months.

References

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History

Etymology

See also

References