View source for Extremely low frequency

{{Short description|The range 3-30 Hz of the electromagnetic spectrum}}
{{Use dmy dates|date=July 2020}}

{{MWband
| freq = 3 to 30 [[Hertz|Hz]]
| wave = 100,000 to 10,000 [[Kilometre|km]], respectively
}}
[[File:Clam Lake ELF.jpg|thumb|upright=1.4|1982 aerial view of the U.S. Navy [[Clam Lake, Wisconsin]], ELF transmitter facility, used to communicate with deeply submerged submarines. The rights of way of the two perpendicular 14&nbsp;mile (23&nbsp;km) overhead transmission lines that constituted the [[ground dipole]] antenna which radiated the ELF waves can be seen at lower left. ]]

'''Extremely low frequency''' ('''ELF''') is the [[International Telecommunication Union|ITU]] designation<ref name="ITU Nomenclature">{{Cite web |title=Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications |url=http://www.itu.int/dms_pubrec/itu-r/rec/v/R-REC-V.431-7-200005-I!!PDF-E.pdf |archive-url=https://web.archive.org/web/20131031020427/http://www.itu.int/dms_pubrec/itu-r/rec/v/R-REC-V.431-7-200005-I%21%21PDF-E.pdf |archive-date=31 October 2013 |access-date=20 February 2013 |publisher=ITU}}</ref> for [[electromagnetic radiation]] ([[radio wave]]s) with [[frequencies]] from 3&nbsp;to&nbsp;30&nbsp;[[Hertz|Hz]], and corresponding [[wavelength]]s of 100,000 to 10,000 kilometers, respectively.<ref>{{Cite web |title=Extremely Low Frequency |url=http://www.nasa.gov/directorates/heo/scan/communications/outreach/funfacts/txt_band_designators_prt.htm |access-date=28 September 2013 |website=ANL Glossary |publisher=NASA |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304211614/http://www.nasa.gov/directorates/heo/scan/communications/outreach/funfacts/txt_band_designators_prt.htm |url-status=dead }}</ref><ref>{{Cite web |title=Extremely low frequency |url=http://www.solareis.anl.gov/glossacro/dsp_wordpopup.cfm?word_id=4668 |archive-url=https://web.archive.org/web/20131029202624/http://www.solareis.anl.gov/glossacro/dsp_wordpopup.cfm?word_id=4668 |archive-date=29 October 2013 |access-date=9 August 2011 |website=ANL Glossary}}</ref> In [[atmospheric science]], an alternative definition is usually given, from 3&nbsp;Hz&nbsp;to&nbsp;3&nbsp;kHz.<ref name="EOS">Liemohn, Michael W. and A. A. CHAN, "[http://lws-trt.gsfc.nasa.gov/trt_liemohn05eos.pdf Unraveling the Causes of Radiation Belt Enhancements]". {{Webarchive|url=https://web.archive.org/web/20100527095828/http://lws-trt.gsfc.nasa.gov/trt_liemohn05eos.pdf |date=27 May 2010 }}. EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION, Volume 88, Number 42, 16 October 2007, pages 427–440. Republished by NASA and accessed online, 8 February 2010. Adobe File, page 2.</ref><ref name="Barr1">{{Cite journal |last1=Barr |first1=R. |last2=Jones |first2=D. Llanwyn |last3=Rodger |first3=C. J. |year=2000 |title=ELF and VLF radio waves |journal=[[Journal of Atmospheric and Solar-Terrestrial Physics]] |volume=62 |issue=17–18 |pages=1689–1718 |bibcode=2000JASTP..62.1689B |doi=10.1016/S1364-6826(00)00121-8}}</ref> In the related [[magnetosphere]] science, the lower-frequency electromagnetic oscillations (pulsations occurring below ~3&nbsp;Hz) are considered to lie in the [[ultra low frequency|ULF]] range, which is thus also defined differently from the [[ITU radio bands]].

ELF radio waves are generated by [[lightning]] and natural disturbances in Earth's [[magnetic field]], so they are a subject of research by atmospheric scientists. Because of the difficulty of building [[Antenna (radio)|antennas]] that can radiate such long waves, ELF have been used in only a very few human-made communication systems. ELF waves can penetrate [[seawater]], which makes them useful in [[communication with submarines]], and a few nations have built military ELF transmitters to transmit signals to their submerged submarines, consisting of huge grounded wire antennas ([[ground dipole]]s) {{Convert|15-60|km||0|abbr=on}} long driven by transmitters producing [[megawatt]]s of power. The United States, Russia, India, and China are the only countries known to have constructed these ELF communication facilities.<ref name="NavyFactSheet">{{Cite web |date=28 June 2001 |title=Extremely Low Frequency Transmitter Site, Clam Lake, Wisconsin |url=http://www.fas.org/nuke/guide/usa/c3i/fs_clam_lake_elf2003.pdf |access-date=17 February 2012 |website=Navy Fact File |publisher=United States Navy}} at the [http://www.fas.org/ Federation of American Scientists website]</ref><ref name="Wolkoff">{{Cite conference |last1=Wolkoff |first1=E. A. |last2=W. A. Kraimer |date=May 1993 |title=Pattern Measurements of U.S. Navy ELF Antennas |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a267991.pdf |archive-url=https://web.archive.org/web/20140405051228/http://www.dtic.mil/dtic/tr/fulltext/u2/a267991.pdf |url-status=live |archive-date=5 April 2014 |location=Belgium |publisher=AGARD Conference proceedings 28 Sep – 2 October 1992, NATO |pages=26.1–26.10 |access-date=17 February 2012 |book-title=ELF/VLF/LF Radio Propagation and Systems Aspects}}</ref><ref name="Coe">{{Cite book |last=Coe |first=Lewis |url=https://books.google.com/books?id=W1JAeg1PiWIC&q=%22extremely+low+frequency%22+ELF+propagation+waveguide&pg=PA143 |title=Wireless Radio: A brief history |publisher=McFarland |year=2006 |isbn=978-0-7864-2662-1 |location=USA |pages=143–144}}</ref><ref name="Sterling">{{Cite book |last=Sterling |first=Christopher H. |url=https://books.google.com/books?id=RBC2nY1rp5MC&q=zevs |title=Military communications: from ancient times to the 21st century |publisher=ABC-CLIO |year=2008 |isbn=978-1-85109-732-6 |pages=431–432}}</ref><ref name="Bashkuev">{{Cite journal |last1=Bashkuev |first1=Yu. B. |last2=V. B. Khaptanov |last3=A. V. Khankharaev |date=December 2003 |title=Analysis of Propagation Conditions of ELF Radio Waves on the "Zeus"–Transbaikalia Path |journal=[[Radiophysics and Quantum Electronics]] |volume=46 |issue=12 |pages=909–917 |bibcode=2003R&QE...46..909B |doi=10.1023/B:RAQE.0000029585.02723.11 |s2cid=119798336}}</ref><ref name="Trond">{{Cite web |last=Jacobsen |first=Trond |year=2001 |title=ZEVS, The Russian 82 Hz ELF Transmitter |url=http://www.vlf.it/zevs/zevs.htm |access-date=17 February 2012 |website=Radio Waves Below 22 kHz |publisher=Renato Romero webpage}}</ref><ref name="janes">{{Cite web |last=Hardy |first=James |date=28 February 2013 |title=India makes headway with ELF site construction |url=http://www.janes.com/article/11147/india-makes-headway-with-elf-site-construction |archive-url=https://web.archive.org/web/20140223135343/http://www.janes.com/article/11147/india-makes-headway-with-elf-site-construction |archive-date=23 February 2014 |access-date=23 February 2014 |publisher=[[Jane's Information Group|IHS Jane's Defence Weekly]]}}</ref><ref name="toi14">{{Cite web |date=31 July 2014 |title=Navy gets new facility to communicate with nuclear submarines prowling underwater |url=http://timesofindia.indiatimes.com/india/Navy-gets-new-facility-to-communicate-with-nuclear-submarines-prowling-underwater/articleshow/39371121.cms |website=[[The Times of India]]}}</ref> The U.S. facilities were used between 1985 and 2004 but are now decommissioned.<ref name="Sterling" />

==Alternative definitions==
ELF is a ''subradio frequency''.<ref name="NASA2">[https://web.archive.org/web/20161222032031/https://www.grc.nasa.gov/smad-ext/wp-content/uploads/sites/82/ohpm10.pdf NASA.gov], page 8. ">0 to 300 Hz ... Extremely low frequency (ELF)". {{webarchive|url=https://web.archive.org/web/20110721050001/http://smad-ext.grc.nasa.gov/shed/pub/ohpm/ohpm10-non-ion.pdf|date=21 July 2011}}.</ref> Some medical [[peer review]]ed journal articles refer to ELF in the context of "extremely low frequency (ELF) magnetic fields (MF)" with frequencies of 50&nbsp;Hz<ref>{{Cite journal |last1=Legros |first1=A |last2=Beuter |first2=A |year=2006 |title=Individual subject sensitivity to extremely low frequency magnetic field |journal=[[NeuroToxicology (journal)|NeuroToxicology]] |volume=27 |issue=4 |pages=534–46 |doi=10.1016/j.neuro.2006.02.007 |pmid=16620992|bibcode=2006NeuTx..27..534L }}</ref> and 50–80&nbsp;Hz.<ref>ESTECIO, Marcos Roberto Higino and SILVA, Ana Elizabete. [http://www.docguide.com/news/content.nsf/PaperFrameSet?OpenForm&refid=2&id=48dde4a73e09a969852568880078c249&c=&newsid=8525697700573E1885256C000059C721&u=http://www.scielo.br/scielo.php?script=sci_abstract&pid=S0034-89102002000300012&lng=en&nrm=iso&tlng=en&ref=/news/content.nsf/SearchResults?openform&Query=Extremely%20low%20frequency&so=date&id=48dde4a73e09a969852568880078c249&Search_Box=All%20DG Alterações cromossômicas causadas pela radiação dos monitores de vídeo de computadores] {{Webarchive|url=https://web.archive.org/web/20050220113944/http://www.docguide.com/news/content.nsf/PaperFrameSet?OpenForm&refid=2&id=48dde4a73e09a969852568880078c249&c=&newsid=8525697700573E1885256C000059C721&u=http%3A%2F%2Fwww.scielo.br%2Fscielo.php%3Fscript%3Dsci_abstract&pid=S0034-89102002000300012&lng=en&nrm=iso&tlng=en&ref=%2Fnews%2Fcontent.nsf%2FSearchResults%3Fopenform&Query=Extremely%20low%20frequency&so=date&id=48dde4a73e09a969852568880078c249&Search_Box=All%20DG |date=20 February 2005 }}. Rev. Saúde Pública [online]. 2002, vol.&nbsp;36, n.&nbsp;3, pp.&nbsp;330–336. ISSN 0034-8910. Republished by docguide.com. Accessed 8 February 2010.</ref> United States
Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300&nbsp;Hz.<ref name="NASA2" /> The [[WHO|World Health Organization]] (WHO) have used ELF to refer to the concept of "extremely low frequency (ELF) electric and magnetic fields (EMF)".<ref name="WHOFact2">"[https://www.who.int/docstore/peh-emf/publications/facts_press/efact/efs205.html Electromagnetic Fields and Public HealthL - Extremely Low Frequency (ELF)]". Fact Sheet N205. November 1998. [[World Health Organization]]. Accessed 12 February 2010. "ELF fields are defined as those having frequencies up to 300 Hz. ... the electric and magnetic fields act independently of one another and are measured separately."</ref> The WHO also stated that at frequencies between 0 and 300&nbsp;Hz, "the wavelengths in air are very long ({{Convert|6000|km||abbr=on}} at 50&nbsp;Hz and {{Convert|5000|km||abbr=on}} at 60&nbsp;Hz), and, in practical situations, the electric and magnetic fields act independently of one another and are measured separately".<ref name="WHOFact2" />

==Propagation==
[[File:Schumann resonance spectrum.gif|thumb|Typical spectrum of ELF electromagnetic waves in the Earth's atmosphere, showing peaks caused by the [[Schumann resonances]]. The Schumann resonances are the resonant frequencies of the spherical Earth–ionosphere cavity. Lightning strikes cause the cavity to "ring" like a bell, causing peaks in the noise spectrum. The sharp power peak at 50&nbsp;Hz is caused by radiation from global [[electric power grid]]s. The rise of the noise at low frequencies (left side) is radio noise caused by slow processes in the Earth's [[magnetosphere]].]]

Due to their extremely long wavelength, ELF waves can [[diffraction|diffract]] around large obstacles, are not blocked by mountain ranges or the horizon, and can travel around the [[curvature of the Earth]]. ELF and [[very low frequency|VLF]] waves propagate long distances by an Earth–ionosphere waveguide mechanism.<ref name="Barr1" /><ref name="Jursa">{{Cite book |editor-last=Jursa |editor-first=Adolph S. |author=S. Basu |author2=J. Buchau |author3=F. J. Rich |author4=E. J. Weber |author5=E. C. Field |author6=J. L. Heckscher |author7=P. A. Kossey |author8=E. A. Lewis |author9=B. S. Dandekar |author10=L. F. McNamara |author11=E. W. Cliver |author12=G. H. Millman |author13=J. Aarons |author14=J. A. Klobuchar |author15=M. F. Mendillo |chapter=Ionospheric Radio Wave Propagation |chapter-url=http://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/afrl_publications/handbook_1985/Chptr10.pdf |title=Handbook of Geophysics and the Space Environment |edition=4th |date=1985 |publisher=Air Force Geophysics Laboratory, U.S. Air Force |pages=10.25–10.27}}</ref> 
The Earth is surrounded by a layer of [[charged particle]]s ([[ion]]s and [[electron]]s) in the atmosphere at an altitude of about {{Convert|60|km|abbr=on}} at the bottom of the [[ionosphere]], called the [[D&nbsp;layer]], which reflects ELF waves. The space between the conductive Earth's surface and the conductive D&nbsp;layer acts as a parallel-plate [[waveguide]] which confines ELF waves, allowing them to propagate long distances without escaping into space. In contrast to VLF waves, the height of the layer is much less than one wavelength at ELF frequencies, so the only mode that can propagate at ELF frequencies is the [[TEM mode]] in [[vertical polarization]], with the [[electric field]] vertical and the [[magnetic field]] horizontal. ELF waves have extremely low attenuation of 1–2&nbsp;dB per {{Convert|1000|km||abbr=on}},<ref name="Jursa" /><ref name="Barr3">Barr, et al (2000) [http://nova.stanford.edu/~vlf/IHY_Test/Tutorials/SubionosphericVLFPropagation/Papers/Barr_2000.pdf ELF and VLF radio waves] ({{Webarchive|url=https://web.archive.org/web/20140405051019/http://nova.stanford.edu/~vlf/IHY_Test/Tutorials/SubionosphericVLFPropagation/Papers/Barr_2000.pdf |date=5 April 2014 }}), p.&nbsp;1695, 1696 (fig.&nbsp;3).</ref> giving a single transmitter the potential to communicate worldwide.

ELF waves can also travel considerable distances through "lossy" media like earth and seawater, which would absorb or reflect higher-frequency radio waves.

===Schumann resonances===
{{Main|Schumann resonances}}
The attenuation of ELF waves is so low that they can travel completely around the Earth several times before decaying to negligible amplitude, and thus waves radiated from a source in opposite directions circumnavigating the Earth on a [[great circle]] path [[interference (wave propagation)|interfere]] with each other.<ref name="Barr2">Barr, et al. (2000) [http://nova.stanford.edu/~vlf/IHY_Test/Tutorials/SubionosphericVLFPropagation/Papers/Barr_2000.pdf ELF and VLF radio waves] ({{Webarchive|url=https://web.archive.org/web/20140405051019/http://nova.stanford.edu/~vlf/IHY_Test/Tutorials/SubionosphericVLFPropagation/Papers/Barr_2000.pdf |date=5 April 2014 }}), p.&nbsp;1700–1701.</ref> At certain frequencies these oppositely directed waves are [[in phase]] and add (reinforce), causing [[standing wave]]s. In other words, the closed spherical Earth–ionosphere cavity acts as a huge [[cavity resonator]], enhancing ELF radiation at its [[resonant frequency|resonant frequencies]]. These are called [[Schumann resonances]] after German physicist [[Winfried Otto Schumann]], who predicted them in 1952,<ref name="Schumann 1952a">{{Cite journal |last=Schumann |first=W. O. |date=1952 |title=Über die strahlungslosen Eigenschwingungen einer leitenden Kugel, die von einer Luftschicht und einer Ionosphärenhülle umgeben ist |lang=de |journal=Zeitschrift für Naturforschung A |volume=7 |issue=2 |pages=149–154 |bibcode=1952ZNatA...7..149S |doi=10.1515/zna-1952-0202 |s2cid=96060996}}</ref><ref name="Schumann 1952b">{{Cite journal |last=Schumann |first=W. O. |date=1952 |title=Über die Dämpfung der elektromagnetischen Eigenschwingnugen des Systems Erde – Luft – Ionosphäre |lang=de |journal=Zeitschrift für Naturforschung A |volume=7 |issue=3–4 |pages=250–252 |bibcode=1952ZNatA...7..250S |doi=10.1515/zna-1952-3-404 |doi-access=free}}</ref><ref name="Schumann 1952c">{{Cite journal |last=Schumann |first=W. O. |date=1952 |title=Über die Ausbreitung sehr langer elektrischer Wellen um die Signale des Blitzes |lang=de |journal=Nuovo Cimento |volume=9 |issue=12 |pages=1116–1138 |bibcode=1952NCim....9.1116S |doi=10.1007/BF02782924 |s2cid=122643775}}</ref><ref name="Schumann 1954d">{{Cite journal |last1=Schumann |first1=W. O. |last2=König |first2=H. |date=1954 |title=Über die Beobachtung von Atmospherics bei geringsten Frequenzen |lang=de |journal=Naturwissenschaften |volume=41 |issue=8 |pages=183–184 |bibcode=1954NW.....41..183S |doi=10.1007/BF00638174 |s2cid=6546863}}</ref> and were detected in the 1950s. Modeling the Earth–ionosphere cavity with perfectly conducting walls, Schumann calculated the resonances should occur at frequencies of<ref name="Barr2" />
: <math>f_n = 7.49 \sqrt{n(n + 1)}\ \text{Hz}.</math>

The actual frequencies differ slightly from this due to the conduction properties of the ionosphere. The fundamental Schumann resonance is at approximately 7.83&nbsp;Hz, the frequency at which the [[wavelength]] equals the circumference of the Earth, and higher harmonics occur at 14.1, 20.3, 26.4, and 32.4&nbsp;Hz, etc. Lightning strikes excite these resonances, causing the Earth–ionosphere cavity to "ring" like a bell, resulting in a peak in the noise spectrum at these frequencies, so the Schumann resonances can be used to monitor global thunderstorm activity.

Interest in Schumann resonances was renewed in 1993 when E. R. Williams showed a correlation between the resonance frequency and tropical air temperatures, suggesting that the resonance could be used to monitor [[global warming]].<ref name="Williams">{{Cite journal |last=Williams |first=Earle R. |date=22 May 1992 |title=The Schumann resonance: A global tropical thermometer |journal=[[Science (journal)|Science]] |volume=256 |issue=5060 |pages=1184–1187 |bibcode=1992Sci...256.1184W |doi=10.1126/science.256.5060.1184 |pmid=17795213 |s2cid=26708495}}</ref><ref name="Barr2" />

==Submarine communications==
[[Image:Ground dipole ELF antenna.svg|thumb|upright=1.8|A [[ground dipole]] antenna used for transmitting ELF waves, similar to the U.S. Navy Clam Lake antennas, showing how it works. It functions as a huge [[loop antenna]], with the alternating current '''I''' from the transmitter '''P''' passing through an overhead transmission line, then deep in the earth from one [[Ground (electricity)|ground connection]] '''G''' to the other, then through another transmission line back to the transmitter. This creates an alternating magnetic field '''H''', which radiates ELF waves. The [[alternating current]] is shown flowing in one direction only through the loop for clarity.]]

Since ELF radio waves can penetrate seawater deeply, to the operating depths of submarines, a few nations have built naval ELF [[transmitter]]s to [[Communication with submarines|communicate with their submarines]] while submerged. It was reported in 2018 that [[China]] had constructed the world's largest ELF facility roughly the size of [[New York City]] in order to communicate with its submarine forces without requiring them to surface.<ref>{{Cite web |url=https://www.thedrive.com/the-war-zone/25728/chinas-new-york-city-sized-earthquake-warning-system-sounds-more-like-way-to-talk-to-subs |title=China's NYC-Sized 'Earthquake Warning System' Array Sounds More Like a Way to Talk to Submarines |date=31 December 2018 }}</ref> The [[United States Navy]] in 1982 built the first ELF submarine communications facility, two coupled ELF transmitters at [[Clam Lake, Wisconsin]], and [[Republic, Michigan]].<ref>"[http://www.navy.mil/navydata/policy/vision/vis98/vis-p10.html U.S. Navy: Vision...Presence...Power]" ({{Webarchive|url=https://web.archive.org/web/20150420230121/http://www.navy.mil/navydata/policy/vision/vis98/vis-p10.html |date=20 April 2015 }}). SENSORS – Subsurface Sensors. US Navy. Accessed 7 February 2010.</ref> They were shut down in 2004. The [[Russian Navy]] operates an ELF transmitter called [[ZEVS (transmitter)|ZEVS]] (Zeus) at [[Murmansk]] on the [[Kola Peninsula]].<ref>[http://www.vlf.it/zevs/zevs.htm ZEVS, the Russian 82 Hz ELF transmitter].</ref> The [[Indian Navy]] has an ELF communication facility at the [[INS Kattabomman]] naval base to communicate with its [[Arihant-class submarine|Arihant-class]] and [[Akula-class submarine]]s.<ref name="toi14"/><ref>{{Cite web |url=http://www.janes.com/article/11147/india-makes-headway-with-elf-site-construction |author=James Hardy |title=India makes headway with ELF site construction |website=IHS Jane's 360 |date=28 February 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140223135343/http://www.janes.com/article/11147/india-makes-headway-with-elf-site-construction |archive-date=2014-02-23}}</ref>

===Explanation===
Because of its [[electrical conductivity]], [[seawater]] shields submarines from most higher-frequency radio waves, making radio communication with submerged submarines at ordinary frequencies impossible. Signals in the ELF frequency range, however, can penetrate much deeper. Two factors limit the usefulness of ELF communications channels: the low data transmission rate of a few characters per minute and, to a lesser extent, the one-way nature due to the impracticality of installing an antenna of the required size on a submarine (the antenna needs to be of an exceptional size in order to achieve successful communication). Generally, ELF signals have been used to order a submarine to rise to a shallow depth where it could receive some other form of communication.

===Difficulties of ELF communication===
One of the difficulties posed when broadcasting in the ELF frequency range is [[antenna (radio)|antenna]] size, because the length of the antenna must be at least a substantial fraction of the length of the waves. For example, a 3&nbsp;[[hertz|Hz]] signal has a wavelength equal to the distance electromagnetic waves travel through a given medium in one third of a second. When the [[refractive index]] of the medium is greater than one, ELF waves propagate slower than the speed of light in vacuum. As used in military applications, the wavelength is {{convert|299,792|km|abbr=on}} per second divided by 50–85&nbsp;Hz, which equals around {{convert|3500|-|6000|km|abbr=on}} long. This is comparable to the [[Earth]]'s diameter of around {{convert|12742|km|abbr=on}}. Because of this huge size requirement, to transmit internationally using ELF frequencies, the Earth itself forms a significant part of the antenna, and extremely long leads into the ground are necessary. Various means, such as [[electrical lengthening]], are used to construct practical radio stations with smaller sizes.

The United States maintained two sites: in the [[Chequamegon-Nicolet National Forest]], [[Wisconsin]], and in the [[Escanaba River State Forest]], [[Michigan]] (originally named [[Project Sanguine]], then downsized and renamed Project ELF prior to construction), until they were dismantled, beginning in late September 2004. Both sites used long [[electric power transmission|power lines]], so-called [[ground dipole]]s, as leads. These leads were in multiple strands ranging from {{convert|22.5|to|45|km}} long. Because of the inefficiency of this method, considerable amounts of [[electric power|electrical power]] were required to operate the system.

==Other uses==
Transmitters in the 22&nbsp;Hz range are also used in pipeline maintenance, or [[pigging]]. The signal is generated as an alternating magnetic field, and the transmitter is mounted to, or to part of, the "pig", the cleaning device inserted into the pipe. The pig is pushed through a mostly metal pipeline. The ELF signal can be detected through the metal, allowing its location to be detected by receivers located outside of the pipe.<ref>Stéphane Sainson, ''Inspection en ligne des pipelines. Principes et méthodes'' {{in lang|fr}}. Ed. Lavoisier. 2007. {{ISBN|978-2-7430-0972-4}}. 332&nbsp;p.</ref> It is used to check whether a pig has passed a certain location or to locate a stuck pig.

Some radio hobbyists record ELF signals using antennas ranging in size from 18-inch active antennas up to several thousand feet in length taking advantage of fences, highway guard rails, and even decommissioned railroad tracks. They then [[pitch control|replay them at higher speeds]] to more easily observe natural low-frequency fluctuations in the [[Earth science#Earth's electromagnetic field|Earth's electromagnetic field]]. Increasing the playback speed increases the [[pitch (music)|pitch]], bringing the tone into the [[audio frequency]] range.{{Citation needed|date=December 2020}}

Since the 2000s, very low frequencies have been used successfully at sea for oil geophysical prospecting.<ref>Stéphane Sainson, ''Electromagnetic seabed logging, A new tool for geoscientists''. Ed. Springer, 2016</ref>

==Natural sources==
Naturally occurring ELF waves are present on Earth, resonating in the region between [[ionosphere]] and surface seen in [[lightning]] strikes that make electrons in the atmosphere oscillate.<ref name="Titan ELF">{{Cite news |date=1 June 2007 |title=Titan's Mysterious Radio Wave |publisher=Jet Propulsion Laboratory |url=http://saturn.jpl.nasa.gov/news/features/feature20070601c.cfm |access-date=2 June 2007 |archive-url=https://web.archive.org/web/20070603142552/http://saturn.jpl.nasa.gov/news/features/feature20070601c.cfm |archive-date=3 June 2007 |df=dmy-all }} Republished as "[http://www.nasa.gov/mission_pages/cassini/whycassini/cassinif-20070601-02.html Casini - Unlocking Saturn's Secrets - Titan's mysterious radio wave] {{Webarchive|url=https://web.archive.org/web/20101224111201/http://www.nasa.gov/mission_pages/cassini/whycassini/cassinif-20070601-02.html |date=24 December 2010 }}". 22 November 2007. NASA. Accessed 7 February 2010.</ref> Although signals generated from lightning discharges were predominantly VLF, it was found that an observable ELF component (slow tail) followed the VLF component in almost all cases.<ref name="JGR">Tepley, Lee R. "[http://www.agu.org/pubs/crossref/1959/JZ064i012p02315.shtml A Comparison of Sferics as Observed in the Very Low Frequency and Extremely Low Frequency Bands]" ({{Webarchive|url=https://web.archive.org/web/20110605235922/http://www.agu.org/pubs/crossref/1959/JZ064i012p02315.shtml |date=5 June 2011 }}). Stanford Research Institute Menlo Park, California. 10 August 1959. 64(12), 2315–2329. Summary republished by American Geophysical Union. Accessed 13 February 2010.</ref> Also, the fundamental mode of the Earth–ionosphere cavity has the wavelength equal to the circumference of the Earth, which gives a resonance frequency of 7.8&nbsp;Hz. This frequency, and higher resonance modes of 14, 20, 26, and 32&nbsp;Hz, appear as peaks in the ELF spectrum and are called [[Schumann resonance]].

ELF waves have also been tentatively identified on Saturn's moon [[Titan (moon)|Titan]]. Titan's surface is thought to be a poor reflector of ELF waves, so the waves may instead be reflecting from the liquid–ice boundary of a subsurface ocean of water and ammonia, the existence of which is predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of {{cvt|1200|km}} but with an additional layer of charged particles at {{cvt|63|km}}. This splits Titan's atmosphere into two separate resonating chambers. The source of natural ELF waves on Titan is unclear, as there does not appear to be extensive lightning activity.<ref name="Titan ELF" />

Huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by [[magnetar]]s. The [[Crab Pulsar|pulsar in the Crab nebula]] radiates powers of this order at 30&nbsp;Hz.<ref>{{Cite web |title=Pulsars |url=https://www.cv.nrao.edu/course/astr534/Pulsars.html |website=www.cv.nrao.edu |access-date=31 December 2018 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112023711/https://www.cv.nrao.edu/course/astr534/Pulsars.html |url-status=dead }}</ref> Radiation of this frequency is below the [[plasma frequency]] of the [[interstellar medium]], thus this medium is opaque to it, and it cannot be observed from Earth.

==Exposure==
{{See|Electromagnetic radiation and health#Extremely low frequency}}

In [[electromagnetic therapy]] and [[electromagnetic radiation and health]] research, [[electromagnetic spectrum]] frequencies between 0 and 100 hertz are considered extremely low-frequency fields.<ref name="Cleary" /> A common source of exposure of the public to ELF fields is 50 or 60&nbsp;Hz electric and magnetic fields from high-voltage electric [[power transmission line]]s and secondary distribution lines, such as those supplying electricity to residential neighborhoods.<ref name="WHOFact2" /><ref name="WHO" /><ref name="Cleary">Cleary, Stephen F. "Electromagnetic Field: A Danger?". The New Book of Knowledge – Medicine And Health. 1990. p.&nbsp;164–174. {{ISBN|0-7172-8244-9}}.</ref>

==Conspiracy theories==
Since the late 1970s, various conspiracy theories have risen around exposure to ELF electric and magnetic fields (EMF).<ref name=WHO>{{cite report |title=Electromagnetic fields and public health |series=Fact Sheet No.&nbsp;322 |date=June 2007 |publisher=[[World Health Organization]] |url=http://www.who.int/mediacentre/factsheets/fs322/en/index.html |access-date=7 February 2010 |archive-url=https://web.archive.org/web/20070701204347/http://www.who.int/mediacentre/factsheets/fs322/en/index.html |archive-date=2007-07-01 |url-status=dead |df=dmy-all}}</ref>{{cn|reason=this ref, nor the following, link ELF\EMF to Conspiracy theories|date=January 2025}} External ELF magnetic fields induce electric fields and currents in the body, which, at very high field strengths, may cause nerve and muscle stimulation and changes in nerve cell excitability in the central nervous system.<ref>{{Cite journal |last=Kim |first=Ju Hwan |last2=Lee |first2=Jin-Koo |last3=Kim |first3=Hyung-Gun |last4=Kim |first4=Kyu-Bong |last5=Kim |first5=Hak Rim |date=2019-05-01 |title=Possible Effects of Radiofrequency Electromagnetic Field Exposure on Central Nerve System |url=https://pmc.ncbi.nlm.nih.gov/articles/PMC6513191/#b86-bt-27-265 |journal=Biomolecules &amp; Therapeutics |language=en |volume=27 |issue=3 |pages=265–275 |doi=10.4062/biomolther.2018.152 |issn=1976-9148 |archive-url=http://web.archive.org/web/20241225115023/https://pmc.ncbi.nlm.nih.gov/articles/PMC6513191/ |archive-date=2024-12-25}}</ref>

ELF at human-perceivable kV/m levels was said to create an annoying tingling sensation in the areas of the body in contact with clothing, particularly the arms, due to the induction of a surface charge by the ELF. Of the volunteers, 7% described the spark discharges as painful when the subject was well-insulated and touched a grounded object within a 5&nbsp;kV/m field, whereas 50% described a similar spark discharge as painful in a 10&nbsp;kV/m field.<ref>{{cite report |title=Extremely Low Frequency Fields Environmental Health Criteria |series=Monograph No.&nbsp;238 |at=chapter&nbsp;5, page&nbsp;121 |publisher=[[World Health Organization]] |url=http://www.who.int/peh-emf/publications/elf_ehc/en/index.html |archive-url=https://web.archive.org/web/20070629003658/http://www.who.int/peh-emf/publications/elf_ehc/en/index.html |archive-date=2007-06-29 |df=dmy-all}}</ref>

===Leukemia===
There is high uncertainty regarding [[correlation]]s between long-term, low-level exposure to ELF fields and a number of health effects, including [[leukemia]] in children. In October 2005, [[WHO]] convened a task group of scientific experts to assess any risks to health that might exist from "exposure to ELF electric and magnetic fields in the frequency range >0 to 100,000&nbsp;Hz (100&nbsp;kHz) in regards to childhood leukemia".<ref name=WHO/> The long-term, low-level exposure is evaluated as average exposure to residential power-frequency magnetic field above 0.3–0.4&nbsp;[[Tesla (unit)|μT]], and it is estimated that only between 1% and 4% of children live in such conditions.<ref name=WHO/> Subsequently, in 2010, a pooled analysis of epidemiological evidence supported the hypothesis that exposure to power-frequency magnetic fields is related to childhood leukemia.<ref name="kheifets">{{Cite journal |last=Kheifets |first=L. |year=2010 |title=Pooled analysis of recent studies on magnetic fields and childhood leukemia |journal=Br J Cancer |volume=103 |issue=7 |pages=1128–1135 |doi=10.1038/sj.bjc.6605838 |pmc=3039816 |pmid=20877339}}</ref>

No other study has found any evidence to support the hypothesis that ELF exposure is a contributing factor to leukemia in children.<ref>{{cite journal |last1=Salvan |first1=A. |last2=Ranucci |first2=A. |last3=Lagorio |first3=S. |last4=Magnani |first4=C |year=2015 |title=Childhood leukemia and 50&nbsp;Hz magnetic fields: Findings from the Italian SETIL case-control study |journal=Int J Environ Res Public Health |volume=12 |issue=2 |pages=2184–2204 |doi=10.3390/ijerph120202184 |doi-access=free |pmc=4344719 |pmid=25689995}}</ref><ref>{{cite book |last1=Kelfkens |first1=Gert |last2=Pruppers |first2=Mathieu |year=2018 |chapter=Magnetic fields and childhood leukemia; science and policy in the Netherlands |title=Embec & Nbc 2017 |isbn=978-981-10-5121-0 |series=IFMBE Proceedings |volume=65 |pages=498–501 |doi=10.1007/978-981-10-5122-7_125}}</ref>

A 2014 study estimated the cases of childhood leukemia attributable to exposure to ELF magnetic fields in the [[European Union]] (EU27), assuming that correlations seen in epidemiological studies were causal. It reported that around 50–60&nbsp;cases of childhood leukemia might be attributable to ELF magnetic fields annually, corresponding to between ~1.5% and ~2.0% of all incident cases of childhood leukemia occurring in the EU27 each year.<ref name=grellier>{{cite journal |last=Grellier |first=J. |year=2014 |title=Potential health impacts of residential exposures to extremely low frequency magnetic fields in Europe |journal=Environ Int |volume=62 |pages=55–63 |doi=10.1016/j.envint.2013.09.017 |doi-access=free |pmid=24161447|bibcode=2014EnInt..62...55G |hdl=10044/1/41782 |hdl-access=free }}</ref> At present,{{when|date=June 2018}} however, [[ICNIRP]] and IEEE consider the scientific evidence related to possible health effects from long-term, low-level exposure to ELF fields insufficient to justify lowering these quantitative exposure limits. In summary, when all of the studies are evaluated together, the evidence suggesting that EMFs may contribute to an increased risk of cancer is non-existent.<ref>{{cite report |title=Electric and magnetic fields from power lines and electrical appliances |date=25 November 2020 |publisher=Government of Canada |url=https://www.canada.ca/en/health-canada/services/home-garden-safety/electric-magnetic-fields-power-lines-electrical-appliances.html}}</ref><ref>{{cite news |title=Expertise de l'Afsset sur les effets sanitaires des champs électromagnétiques d'extrêmement basses fréquences |lang=fr |date=6 April 2010 |website=afsset.fr |url=http://www.afsset.fr/index.php?pageid=452&newsid=552&MDLCODE=news |access-date=23 April 2010}}</ref> Epidemiological studies suggest a possible [[correlation and dependence|association]] between long-term occupational exposure to ELF and [[Alzheimer's disease]].<ref name="Garcia2008">{{cite journal |author1=García A. M. |author2=Sisternas A. |author3=Hoyos S. P. |date=April 2008 |title=Occupational exposure to extremely low frequency electric and magnetic fields and Alzheimer disease: a meta-analysis |journal=[[International Journal of Epidemiology]] |volume=37 |issue=2 |pages=329–340 |doi=10.1093/ije/dym295 |doi-access=free |pmid=18245151}}</ref><ref>{{cite report |author=Scientific Committee on Emerging Newly Identified Health Risks-SCENIHR |date=January 2009 |title=Health effects of exposure to EMF |series=European Directorate General for Health & Consumers |place=Brussels, Belgium |publisher=European Commission |pages=4–5 |url=http://ec.europa.eu/health/ph_risk/committees/04_scenihr/docs/scenihr_o_022.pdf |access-date=27 April 2010}}</ref>

===Ecological impact===
There have been some concerns over the possible ecological impact of ELF signals. In 1984 a federal judge halted construction,{{of what|date=December 2021}} requiring more environmental and health studies. This judgment was overruled by a federal appeals court on the basis that the [[US Navy]] claimed to have spent over $25 million studying the effects of the electromagnetic fields, with results indicating that they were similar to the effect produced by standard power-distribution lines. However, during the time that ELF was in use, some Wisconsin politicians, such as Democratic Senators [[Herb Kohl]], [[Russ Feingold]] and Congressman [[Dave Obey]], continued to call for its closure.

Extremely low frequency (ELF) electromagnetic fields (EMFs), typically ranging from 0.3 Hz to 300 Hz, have various ecological impacts on both flora and fauna.<ref>{{Cite journal |last1=Levitt |first1=B. Blake |last2=Lai |first2=Henry C. |last3=Manville |first3=Albert M. |date=2022-11-25 |title=Low-level EMF effects on wildlife and plants: What research tells us about an ecosystem approach |journal=Frontiers in Public Health |language=English |volume=10 |doi=10.3389/fpubh.2022.1000840 |doi-access=free |issn=2296-2565 |pmc=9732734 |pmid=36505009}}</ref>

==Patents==
* Tanner, R. L., ''{{US patent|3215937}}'', "''Extremely low-frequency antenna''", 1965
* Hansell, Clarence W., ''{{US patent|2389432}}'', "''Communication system by pulses through the Earth''"
* Altshuler, {{US patent|4051479}}, ''ELF vertical dipole antenna suspended from aircraft''

==See also==
*[[Ultra low frequency]]
*[[List of initialisms]]
*[[Infrasound]]
*[[Skin effect]]
*[[TACAMO]]
*[[Wardenclyffe Tower]]
*[[Naval Communication Station Harold E. Holt]]
*[[Magnetic pulsations]]

==References==

===Notes===
{{Reflist}}

===General information===
* [[Non-ionizing radiation]], Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the [[International Agency for Research on Cancer|IARC]]. ([http://monographs.iarc.fr/ENG/Monographs/vol80/mono80.pdf Non-Ionizing Radiation])

==External links==
* Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''". IK1QFK Home Page (vlf.it).
* [http://www.inchem.org/documents/ehc/ehc/ehc35.htm Extremely low frequency (ELF) fields] (EHC 35, 1984)
* "''[http://www.vlf.it/ Radio waves below 22kHz]: Nature's signals and strange emission at very low frequency''" - a site specialising in low-frequency signals .
* Jacobsen, Trond, "''[http://www.vlf.it/zevs/zevs.htm ZEVS, the Russian 82 Hz ELF transmitter]: An Extrem Low Frequency transmission-system, using the real longwaves''" ALFLAB, Halden, Norway.
* [http://spaceweather.com/glossary/inspire.html NASA live streaming ELF -> VLF Receiver]

{{radio_spectrum}}
{{EMSpectrum}}
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{{DEFAULTSORT:Extremely Low Frequency}}
[[Category:Radio spectrum]]
[[Category:IARC Group 2B carcinogens]]