Extremely low frequency

Extremely low frequency
Frequency range
 3 to 30 Hz
Wavelength range
 100,000 to 10,000 km, respectively
1982 aerial view of the U.S. Navy Clam Lake, Wisconsin ELF transmitter facility, used to communicate with deeply submerged submarines.

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. The US, Russia, and India are the only nations known to have constructed 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] ELF waves can also penetrate significant distances into earth or rock, and "through-the-earth" underground mine communication systems use frequencies of 300 to 3000 Hz. The frequency of alternating current flowing in electric power grids, 50 or 60 Hz, also falls within the ELF band, making power grids an unintentional source of ELF radiation.

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 (6000 km at 50 Hz and 5000 km 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 Schuman resonances are the resonant frequencies of the spherical Earth-ionosphere cavity. Lighning 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, and 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) in the atmosphere at an altitude of about 60 km 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 1000 km,.[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

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, 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]

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.[21][20]

Submarine communications

The United States Navy utilized extremely low frequencies (ELFs) as radio band and radio communications. The Submarine Integrated Antenna System (SIAS) was a research and development effort to communicate with submerged submarines.[22] The Soviet/Russian Navy also utilized ELFs for submarine communications system, ZEVS.[23] The Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines.[24][25]

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 were 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. Taking account of refractive index, ELF waves propagate slightly 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 US 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 dollars 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 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 found in pipeline inspection gauges, also known as "PIGs". The signal is generated as an alternating magnetic field, the transmitter is mounted to or part of the PIG. The PIG is pushed through a pipeline, mostly made of metal. The ELF signal can be detected through the metal on the outside.[26] It is needed to check if a PIG has passed a certain location and to locate a stuck PIG.

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. They are initiated by lightning strikes that make electrons in the atmosphere oscillate.[27] 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.[28] 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.[27]

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 the frequency 30 hertz.[29] 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.[30] A common source of exposure of the public to ELF fields is 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][31][30]

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.[31] There are established biological effects from acute exposure at high levels (well above 100 µT) that are explained by recognized biophysical mechanisms. 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). 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.[32] 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.[33]

There is some uncertainty regarding possible associations between long-term, low-level exposure to ELF fields and a number of health effects, including leukaemia 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 leukaemia."[31] 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.[31] Subsequently, in 2010, a pooled analysis of epidemiological evidence supported the hypothesis that exposure to power frequency magnetic fields is related to childhood leukaemia.[34]

A 2014 study estimated the cases of childhood leukaemia attributable to exposure to ELF magnetic fields in the European Union (EU27), assuming that associations seen in epidemiological studies were causal. It reported that around 50-60 cases of childhood leukaemia might be attributable to ELF magnetic fields annually, corresponding to between ~1.5% and ~2.0% of all incident cases of childhood leukaemia occurring in the EU27 each year.[35] 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 very weak.[36][37] Epidemiological studies suggest a possible association between long term occupational exposure to ELF and Alzheimer's disease.[38][39]

Patents

See also

References

Notes

  1. ↑ "Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications" (PDF). ITU. Retrieved 20 February 2013.
  2. ↑ "Extremely Low Frequency". ANL Glossary. NASA. Retrieved 28 September 2013.
  3. ↑ "Extremely low frequency". ANL Glossary. Retrieved 9 August 2011.
  4. ↑ Liemohn, Michael W. and A. A. CHAN, "Unraveling the Causes of Radiation Belt Enhancements". 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.
  5. 1 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.
  6. ↑ "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
  7. ↑ 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 Sept. – 2 Oct. 1992, NATO. pp. 26.1–26.10. Retrieved 17 February 2012.
  8. ↑ Coe, Lewis (2006). Wireless Radio: A brief history. USA: McFarland. pp. 143–144. ISBN 0786426624.
  9. 1 2 Sterling, Christopher H. (2008). Military communications: from ancient times to the 21st century. ABC-CLIO. pp. 431–432. ISBN 1851097325.
  10. ↑ 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. Plenum. 46 (12): 909–917. Bibcode:2003R&QE...46..909B. doi:10.1023/B:RAQE.0000029585.02723.11. Retrieved 17 February 2012.
  11. ↑ Jacobsen, Trond (2001). "ZEVS, The Russian 82 Hz ELF Transmitter". Radio Waves Below 22 kHz. Renato Romero webpage. Retrieved 17 February 2012.
  12. ↑ 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.
  13. ↑ "Navy gets new facility to communicate with nuclear submarines prowling underwater". The Times of India. 31 July 2014.
  14. 1 2 NASA.gov, page 8. ">0 to 300 Hz ... Extremely low frequency (ELF)" Archived 21 July 2011 at the Wayback Machine.
  15. ↑ Legros, A; Beuter, A (2006). "Individual subject sensitivity to extremely low frequency magnetic field". Neurotoxicology. 27 (4): 534–46. PMID 16620992. doi:10.1016/j.neuro.2006.02.007.
  16. ↑ ESTECIO, Marcos Roberto Higino and SILVA, Ana Elizabete. Alterações cromossômicas causadas pela radiação dos monitores de vídeo de computadores. Rev. Saúde Pública [online]. 2002, vol.36, n.3, pp. 330-336. ISSN 0034-8910. Republished by docguide.com. Accessed 8 February 2010.
  17. 1 2 3 "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."
  18. 1 2 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.
  19. ↑ Barr, et al (2000) ELF and VLF radio waves, p. 1695, 1696 fig. 3
  20. 1 2 3 Barr, et al (2000) ELF and VLF radio waves, p. 1700-1701
  21. ↑ Williams, Earle R. (May 22, 1992). "The Schumann resonance: A global tropical thermometer". Science. AAAS. 256 (5060): 1184–1187. doi:10.1126/science.256.5060.1184. Retrieved February 27, 2017.
  22. ↑ "U.S. Navy: Vision...Presence...Power." SENSORS - Subsurface Sensors. US Navy. Accessed 7 February 2010.
  23. ↑ http://www.vlf.it/zevs/zevs.htm ZEVS, the Russian 82 Hz ELF transmitter
  24. ↑ "Navy gets new facility to communicate with nuclear submarines prowling underwater". The Times of India. 31 July 2014.
  25. ↑ http://www.janes.com/article/11147/india-makes-headway-with-elf-site-construction
  26. ↑ Stéphane Sainson, Inspection en ligne des pipelines. Principes et méthodes. Ed. Lavoisier. 2007. ISBN 978-2743009724. 332 p.
  27. 1 2 "Titan's Mysterious Radio Wave". Jet Propulsion Laboratory. 1 June 2007. Retrieved 2007-06-02. Republished as "Casini - Unlocking Saturn's Secrets - Titan's mysterious radio wave". 22 November 2007. NASA. Accessed 7 February 2010.
  28. ↑ 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
  29. ↑ http://www.cv.nrao.edu/course/astr534/Pulsars.html
  30. 1 2 Cleary, Stephen F. "Electromagnetic Field: A Danger?". The New Book of Knowledge - Medicine And Health. 1990. 164-74. ISBN 0-7172-8244-9.
  31. 1 2 3 4 "Electromagnetic fields and public health". Fact Sheet No. 322, June 2007. World Health Organization, Accessed 7 February 2010.
  32. ↑ 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.
  33. ↑ Extremely Low Frequency Fields Environmental Health Criteria Monograph No.238, chapter 5, page 121, WHO
  34. ↑ Kheifets, L (2010). ""Pooled analysis of recent studies on magnetic fields and childhood leukaemia".". Brit J Cancer. 103: 1128–1135.
  35. ↑ Grellier, J (2014). ""Potential health impacts of residential exposures to extremely low frequency magnetic fields in Europe".". Environ Int. 62: 55–63.
  36. ↑ "Electric and magnetic fields from power lines and electrical appliances". Government of Canada.
  37. ↑ "Expertise de l'Afsset sur les effets sanitaires des champs électromagnétiques d'extrêmement basses fréquences" (in French). 6 April 2010. Retrieved 23 April 2010.
  38. ↑ 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. PMID 18245151. doi:10.1093/ije/dym295.
  39. ↑ 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 2010-04-27.

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