The Heaviside layer, sometimes called the Kennelly–Heaviside layer, named after Arthur E. Kennelly and Oliver Heaviside, is a layer of ionised gas occurring roughly between 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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 (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...

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

horizon The horizon is the apparent curve 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 curve divides all viewing directions based on whethe ...

. 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 curvatur ...

" 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 The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. Conversely, on the night ( lee) 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 axial tilt, tilted orbit around the Sun. In temperat ...

, and the amount of

sunspot Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...

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

Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquess of Marconi ( ; ; 25 April 1874 – 20 July 1937) was an Italian electrical engineer, inventor, and politician known for his creation of a practical radio wave-based Wireless telegraphy, wireless tel ...

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 The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

.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 In physics, total internal reflection (TIR) is the 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 refl ...

, 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 ( ''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 The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, t ...

'' and the ''

group velocity The group velocity of a wave is the velocity with which the overall envelope shape of the wave's amplitudes—known as the ''modulation'' or ''envelope (waves), envelope'' of the wave—propagates through space. For example, if a stone is thro ...

''. 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 of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity, "On the Ele ...

, 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 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

{{DEFAULTSORT:Kennelly-Heaviside layer Ionosphere Radio frequency propagation ru:Ионосфера#Слой Е

History

Etymology

See also

References