Extremely low frequency (ELF) is the ITU designation[1] for electromagnetic radiation (radio waves) with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively.[2][3] In atmospheric science, an alternative definition is usually given, from 3 Hz to 3 kHz.[4][5] In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the 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 antennas that can radiate such long waves, ELF frequencies 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 dipoles) 15–60 km (9–37 mi) long driven by transmitters producing megawatts of power. The United States, Russia, India, and China are the only nations known to have constructed these ELF communication facilities.[6][7][8][9][10][11][12][13] The U.S. facilities were used between 1985 and 2004 but are now decommissioned.[9]
Alternate definitions
ELF is a subradio frequency.[14] Some medical peer reviewed journal articles refer to ELF in the context of "extremely low frequency (ELF) magnetic fields (MF)" with frequencies of 50 Hz[15] and 50–80 Hz.[16] United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz.[14] The World Health Organization (WHO) have used ELF to refer to the concept of "extremely low frequency (ELF) electric and magnetic fields (EMF)"[17] The WHO also stated that at frequencies between 0 and 300 Hz, "the wavelengths in air are very long (6,000 km (3,700 mi) at 50 Hz and 5,000 km (3,100 mi) at 60 Hz), and, in practical situations, the electric and magnetic fields act independently of one another and are measured separately."[17]
Propagation
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 Hz is caused by radiation from global electric power grids. 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 diffract around large obstacles, are not blocked by mountain ranges or the horizon, and can travel around the curve of the Earth. ELF and VLF waves propagate long distances by an Earth-ionosphere waveguide mechanism.[5][18] The Earth is surrounded by a layer of charged particles (ions and electrons) in the atmosphere at an altitude of about 60 km (37 mi) at the bottom of the ionosphere, called the D layer which reflects ELF waves. The space between the conductive Earth's surface and the conductive D 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 dB per 1,000 km (620 mi),[18][19] 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 article: 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 interfere with each other.[20] At certain frequencies these oppositely directed waves are in phase and add (reinforce), causing standing waves. In other words, the closed spherical Earth-ionosphere cavity acts as a huge cavity resonator, enhancing ELF radiation at its resonant frequencies. These are called Schumann resonances after German physicist Winfried Otto Schumann who predicted them in 1952,[21][22][23][24] and were detected in the 1950s. Modeling the Earth-ionosphere cavity with perfectly conducting walls, Schumann calculated the resonances should occur at frequencies of[20]
\( {\displaystyle f_{n}=7.49[n(n+1)]^{1/2}\;{\text{Hz}}} \)
The actual frequencies differ slightly from this due to the conduction properties of the ionosphere. The fundamental Schumann resonance is at approximately 7.83 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 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 the resonance could be used to monitor global warming.[25][20]
Submarine communications
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 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 transmitters to communicate with their submarines while submerged. China has recently 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.[26] The United States Navy in 1982 built the first ELF submarine communications facility, two coupled ELF transmitters at Clam Lake, Wisconsin and Republic, Michigan.[27] They were shut down in 2004. The Russian Navy operates an ELF transmitter called ZEVS (Zeus) at Murmansk on the Kola Peninsula.[28] The Indian Navy has an ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines.[29][30]
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 size, because the length of the antenna must be at least a substantial fraction of the length of the waves. Simply put, a 3 Hz (cycle per second) signal would have a wavelength equal to the distance EM 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 a vacuum. As used in military applications, the wavelength is 299,792 km (186,282 mi) per second divided by 50–85 Hz, which equals around 3,500 to 6,000 km (2,200 to 3,700 mi) long. This is comparable to the Earth's diameter of around 12,742 km (7,918 mi). 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 are necessary into the ground. 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 rechristened Project ELF prior to construction), until they were dismantled, beginning in late September 2004. Both sites used long power lines, so-called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometres (14.0 to 28.0 mi) long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.
Ecological impact
There have been some concerns over the possible ecological impact of ELF signals. In 1984 a federal judge halted construction, 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. The judgment was not accepted by everyone and, during the time that ELF was in use, some Wisconsin politicians such as Democratic Senators Herb Kohl, Russ Feingold and Congressman Dave Obey called for its closure. Similar concerns have, in the past, been raised about electromagnetic radiation and health.
Other uses
Transmitters in the 22 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 pipeline mostly made of metal. The ELF signal can be detected through the metal allowing its location to be detected by receivers located outside of the pipe.[31] It is needed to check if a pig has passed a certain location and to locate a pig which has become stuck.
Some radio monitoring hobbyists record ELF signals using antennas ranging in size from eighteen inch active antennas up to several thousand feet in length taking advantage of fences, highway guard rails, and even decommissioned railroad tracks, and play them back at higher speeds to more easily observe natural low frequency fluctuations in the Earth's electromagnetic field. Increasing the playback speed increases the pitch, so that it can be brought into the audio frequency range for audibility.
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.[32] Though VLF signals were predominantly generated from lightning discharges, it was found that an observable ELF component—slow tail—followed the VLF component in almost all cases.[33] 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 Hz. This frequency, and higher resonance modes of 14, 20, 26 and 32 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. Titan's surface is thought to be a poor reflector of ELF waves, so the waves may instead be reflecting off 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 1,200 km (750 mi) but with an additional layer of charged particles at 63 km (39 mi). 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.[32]
Huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by magnetars. The pulsar in the Crab nebula radiates powers of this order at 30 Hz.[34] 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
In electromagnetic therapy and electromagnetic radiation and health research, electromagnetic spectrum frequencies between 0 and 100 hertz are considered extremely low-frequency fields.[35] A common source of exposure of the public to ELF fields is 50 Hz / 60 Hz electric and magnetic fields from high-voltage electric power transmission lines and secondary distribution lines, such as those supplying electricity to residential neighborhoods.[17][36][35]
Possible health effects
Since the late 1970s, questions have been raised whether exposure to ELF electric and magnetic fields (EMF) within this range of frequencies produces adverse health consequences.[36] External ELF magnetic fields induce electric fields and currents in the body which, at very high field strengths, cause nerve and muscle stimulation and changes in nerve cell excitability in the central nervous system. Health effects related to short-term, high-level exposure have been established and form the basis of two international exposure limit guidelines (ICNIRP, 1998; IEEE, 2002) such as 0.2-0.4 mA at 50/60 Hz. A study by Reilly in 1999 showed that the threshold for direct perception of exposure to ELF RF by human volunteer subjects started at around 2 to 5 kV/m at 60 Hz, with 10% of volunteers detecting the ELF exposure at this level. The percentage of detection increased to 50% of volunteers when the ELF level was raised from 7 to 20 kV/m. 5% of all test subjects considered the perception of ELF at these thresholds annoying.[37] 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. 7% of volunteers described the spark discharges as painful where the subject was well-insulated and touched a grounded object within a 5 kV/m field. 50% of volunteers described a similar spark discharge as painful in a 10 kV/m field.[38]
Leukemia
There is high uncertainty regarding correlations 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 Hz (100 kHz) in regards to childhood leukemia."[36] The long-term, low-level exposure is evaluated as average exposure to residential power-frequency magnetic field above 0.3 to 0.4 µT, and it is estimated that only between 1% and 4% of children live in such conditions.[36] Subsequently, in 2010, a pooled analysis of epidemiological evidence supported the hypothesis that exposure to power frequency magnetic fields is related to childhood leukemia.[39] No other study has found any evidence to support the hypothesis that ELF exposure is a contributing factor to leukemia in children.[40][41]
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 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.[42] At present, 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.[43][44] Epidemiological studies suggest a possible association between long term occupational exposure to ELF and Alzheimer's disease.[45][46]
Patents
Tanner, R. L., U.S. Patent 3,215,937, "Extremely low-frequency antenna", 1965
Hansell, Clarence W., U.S. Patent 2,389,432, "Communication system by pulses through the Earth"
Altshuler, U.S. Patent 4,051,479, 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
References
Notes
"Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications" (PDF). ITU. Archived from the original (PDF) on 31 October 2013. Retrieved 20 February 2013.
"Extremely Low Frequency". ANL Glossary. NASA. Retrieved 28 September 2013.
"Extremely low frequency". ANL Glossary. Archived from the original on 29 October 2013. Retrieved 9 August 2011.
Liemohn, Michael W. and A. A. CHAN, "Unraveling the Causes of Radiation Belt Enhancements Archived 27 May 2010 at the Wayback Machine". 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.
Barr, R.; Jones, D. Llanwyn; Rodger, C. J. (2000). "ELF and VLF radio waves". Journal of Atmospheric and Solar-Terrestrial Physics. 62 (17–18): 1689–1718. Bibcode:2000JASTP..62.1689B. doi:10.1016/S1364-6826(00)00121-8.
"Extremely Low Frequency Transmitter Site, Clam Lake, Wisconsin" (PDF). Navy Fact File. United States Navy. 28 June 2001. Retrieved 17 February 2012. at the Federation of American Scientists website
Wolkoff, E. A.; W. A. Kraimer (May 1993). "Pattern Measurements of U.S. Navy ELF Antennas" (PDF). ELF/VLF/LF Radio Propagation and Systems Aspects. Belgium: AGARD Conference proceedings 28 Sep – 2 October 1992, NATO. pp. 26.1–26.10. Retrieved 17 February 2012.
Coe, Lewis (2006). Wireless Radio: A brief history. USA: McFarland. pp. 143–144. ISBN 978-0786426621.
Sterling, Christopher H. (2008). Military communications: from ancient times to the 21st century. ABC-CLIO. pp. 431–432. ISBN 978-1851097326.
Bashkuev, Yu. B.; V. B. Khaptanov; A. V. Khankharaev (December 2003). "Analysis of Propagation Conditions of ELF Radio Waves on the "Zeus"–Transbaikalia Path". Radiophysics and Quantum Electronics. 46 (12): 909–917. Bibcode:2003R&QE...46..909B. doi:10.1023/B:RAQE.0000029585.02723.11.
Jacobsen, Trond (2001). "ZEVS, The Russian 82 Hz ELF Transmitter". Radio Waves Below 22 kHz. Renato Romero webpage. Retrieved 17 February 2012.
Hardy, James (28 February 2013). "India makes headway with ELF site construction". IHS Jane's Defence Weekly. Archived from the original on 23 February 2014. Retrieved 23 February 2014.
"Navy gets new facility to communicate with nuclear submarines prowling underwater". The Times of India. 31 July 2014.
NASA.gov, page 8. ">0 to 300 Hz ... Extremely low frequency (ELF)" Archived 21 July 2011 at the Wayback Machine
Legros, A; Beuter, A (2006). "Individual subject sensitivity to extremely low frequency magnetic field". NeuroToxicology. 27 (4): 534–46. doi:10.1016/j.neuro.2006.02.007. PMID 16620992.
ESTECIO, Marcos Roberto Higino and SILVA, Ana Elizabete. Alterações cromossômicas causadas pela radiação dos monitores de vídeo de computadores Archived 20 February 2005 at the Wayback Machine. Rev. Saúde Pública [online]. 2002, vol.36, n.3, pp. 330-336. ISSN 0034-8910. Republished by docguide.com. Accessed 8 February 2010.
"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."
Jursa, Adolph S., Ed. (1985). Handbook of Geophysics and the Space Environment, 4th Ed (PDF). Air Force Geophysics Laboratory, U.S. Air Force. pp. 10.25–10.27.
Barr, et al (2000) ELF and VLF radio waves, p. 1695, 1696 fig. 3
Barr, et al (2000) ELF and VLF radio waves, p. 1700-1701
Schumann, W. O. (1952). "Über die strahlungslosen Eigenschwingungen einer leitenden Kugel, die von einer Luftschicht und einer Ionosphärenhülle umgeben ist". Zeitschrift für Naturforschung A. 7 (2): 149–154. Bibcode:1952ZNatA...7..149S. doi:10.1515/zna-1952-0202.
Schumann, W. O. (1952). "Über die Dämpfung der elektromagnetischen Eigenschwingnugen des Systems Erde – Luft – Ionosphäre". Zeitschrift für Naturforschung A. 7 (3–4): 250–252. Bibcode:1952ZNatA...7..250S. doi:10.1515/zna-1952-3-404.
Schumann, W. O. (1952). "Über die Ausbreitung sehr Langer elektriseher Wellen um die Signale des Blitzes". Nuovo Cimento. 9 (12): 1116–1138. Bibcode:1952NCim....9.1116S. doi:10.1007/BF02782924.
Schumann, W. O.; König, H. (1954). "Über die Beobactung von Atmospherics bei geringsten Frequenzen". Naturwissenschaften. 41 (8): 183–184. Bibcode:1954NW.....41..183S. doi:10.1007/BF00638174.
Williams, Earle R. (22 May 1992). "The Schumann resonance: A global tropical thermometer". Science. 256 (5060): 1184–1187. Bibcode:1992Sci...256.1184W. doi:10.1126/science.256.5060.1184. PMID 17795213.
https://www.thedrive.com/the-war-zone/25728/chinas-new-york-city-sized-earthquake-warning-system-sounds-more-like-way-to-talk-to-subs
"U.S. Navy: Vision...Presence...Power." SENSORS - Subsurface Sensors. US Navy. Accessed 7 February 2010.
http://www.vlf.it/zevs/zevs.htm ZEVS, the Russian 82 Hz ELF transmitter
"Navy gets new facility to communicate with nuclear submarines prowling underwater". The Times of India. 31 July 2014.
http://www.janes.com/article/11147/india-makes-headway-with-elf-site-construction
Stéphane Sainson, Inspection en ligne des pipelines. Principes et méthodes. Ed. Lavoisier. 2007. ISBN 978-2743009724. 332 p.
"Titan's Mysterious Radio Wave". Jet Propulsion Laboratory. 1 June 2007. Archived from the original on 3 June 2007. Retrieved 2 June 2007. Republished as "Casini - Unlocking Saturn's Secrets - Titan's mysterious radio wave". 22 November 2007. NASA. Accessed 7 February 2010.
Tepley, Lee R. "A Comparison of Sferics as Observed in the Very Low Frequency and Extremely Low Frequency Bands". Stanford Research Institute Menlo Park, California. 10 August 1959. 64(12), 2315–2329. Summary republished by American Geophysical Union. Accessed 13 February 2010
"Pulsars". www.cv.nrao.edu.
Cleary, Stephen F. "Electromagnetic Field: A Danger?". The New Book of Knowledge - Medicine And Health. 1990. 164-74. ISBN 0-7172-8244-9.
"Electromagnetic fields and public health". Fact Sheet No. 322, June 2007. [World Health Organization], Accessed 7 February 2010. (archive link)
Reilly, JP (1999). "Comments concerning "Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 GHz)"". Health Phys. 76 (3): 314–315.
Extremely Low Frequency Fields Environmental Health Criteria Monograph No.238, chapter 5, page 121, WHO
Kheifets, L (2010). ""Pooled analysis of recent studies on magnetic fields and childhood leukemia"". Br J Cancer. 103 (7): 1128–1135. doi:10.1038/sj.bjc.6605838. PMC 3039816. PMID 20877339.
Salvan, A; Ranucci, A; Lagorio, S; Magnani, C (2015). "Childhood Leukemia and 50 Hz Magnetic Fields: Findings from the Italian SETIL Case-Control Study". Int J Environ Res Public Health. 12 (2): 2184–204. doi:10.3390/ijerph120202184. PMC 4344719. PMID 25689995.
Kelfkens, Gert; Pruppers, Mathieu (2018). "Magnetic fields and childhood leukemia; science and policy in the Netherlands". Embec & Nbc 2017. IFMBE Proceedings. 65. pp. 498–501. doi:10.1007/978-981-10-5122-7_125. ISBN 978-981-10-5121-0.
Grellier, J (2014). ""Potential health impacts of residential exposures to extremely low frequency magnetic fields in Europe"". Environ Int. 62: 55–63. doi:10.1016/j.envint.2013.09.017. PMID 24161447.
"Electric and magnetic fields from power lines and electrical appliances". Government of Canada.
"Expertise de l'Afsset sur les effets sanitaires des champs électromagnétiques d'extrêmement basses fréquences" (in French). 6 April 2010. Retrieved 23 April 2010.
García AM, Sisternas A, Hoyos SP (April 2008). "Occupational exposure to extremely low frequency electric and magnetic fields and Alzheimer disease: a meta-analysis". International Journal of Epidemiology. 37 (2): 329–40. doi:10.1093/ije/dym295. PMID 18245151.
Scientific Committee on Emerging; Newly Identified Health Risks-SCENIHR (January 2009). "Health Effects of Exposure to EMF" (PDF). Brussels: Directorate General for Health & Consumers - European Commission: 4–5. Retrieved 27 April 2010.
General information
Non-ionizing radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC. (Non-Ionizing Radiation)
External links
Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
Extremely low frequency (ELF) fields (EHC 35, 1984)
"Radio waves below 22kHz: Nature's signals and strange emission at very low frequency" - a site specialising in low-frequency signals .
Jacobsen, Trond, "ZEVS, the Russian 82 Hz ELF transmitter: An Extrem Low Frequency transmission-system, using the real longwaves" ALFLAB, Halden, Norway.
NASA live streaming ELF -> VLF Receiver
Hellenica World - Scientific Library
Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License
