Extremely low frequency
Extremely low frequency (ELF) waves are electromagnetic radiation (radio waves) with frequencies from 3 to 30 Hz, and corresponding wavelengths from 100,000 to 10,000 kilometers.  In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz. 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 man-made communication systems. ELF waves can penetrate seawater, so the US and Russian militaries have used ELF transmission facilities to communicate with their submerged submarines. The frequency of alternating current flowing in electric power grids, 50 or 60 Hz, falls within this frequency band, and power grids are an unintentional source of ELF radiation.
ELF is a subradio frequency. 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 and 50–80 Hz. United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz. The World Health Organization (WHO) have used ELF to refer to the concept of "extremely low frequency (ELF) electric and magnetic fields (EMF)" and have also referred to "ELF electric and magnetic fields in the frequency range >0 to 100,000 Hz (100 kHz)." 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."
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. The Soviet/Russian Navy also utilized ELFs for submarine communications system, ZEVS.
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 (antennas need to be of exceptional size for the users 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 3,730 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 taken to construct practical radio stations with smaller sizes.
The US maintained two sites, in the Chequamegon-Nicolet National Forest, Wisconsin and 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.
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 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.
Transmitters in the 20 Hz range are also found in pipeline inspection gauges, also known as "PIGs".
Some radio hams record ELF (or even lower) signals from very large homemade antennas, and play them back at higher speeds to catch natural fluctuations in the Earth's electromagnetic field. Increasing the playback increases the pitch, so that it is brought into the audio frequency range.
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. 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. 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.
They 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 doesn't appear to be extensive lightning activity.
Finally, 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. 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.
In electromagnetic therapy and electromagnetic radiation and health research, electromagnetic spectrum frequencies between 0 and 100 hertz are considered extremely low-frequency fields. 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. 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." 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). At present, these bodies 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. 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. A common source of ELF fields in the United States is 60 Hz electric and magnetic fields from high-voltage electric power transmission lines and secondary distribution lines, such as those found in residential neighborhoods. The International Agency for Research on Cancer (IARC) has evaluated the scientific data and has classified ELF magnetic fields as being "possibly carcinogenic" to humans. IARC based this classification on the following:
Human health population studies showing weak evidence of an association with childhood leukemia; and a large database of laboratory study results showing inadequate evidence of an association with cancer in animals.
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. Epidemiological studies suggest a possible association between long term occupational exposure to ELF and Alzheimer's disease.
ELF Effects on human nervous system
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 expose at this level. The percent of detection raised to 50% of volunteers when the ELF level was rasised to 7 to 20 kV/m. 5% of all test subjects considered the perception of ELF at these thresholds as annoying.
ELF at human perceivable kV/m levels is 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.
- 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
- Communication with submarines
- Electromagnetic spectrum
- Ground dipole
- List of initialisms
- Schumann resonance
- Skin effect
- Wardenclyffe Tower
- NASA "Extremely Low Frequency". ANL Glossary. Retrieved 28 September 2013.
- "Extremely low frequency". ANL Glossary. Retrieved 9 August 2011.
- 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.
- R. Barr, D. Llanwyn Jones, C. J. Rodger, "ELF and VLF radio waves", Journal of Atmospheric and Solar-Terrestrial Physics, Volume 62, Issues 17-18, November 2000, Pages 1689-1718, ISSN 1364-6826, doi:10.1016/S1364-6826(00)00121-8.
- NASA.gov, page 8. ">0 to 300 Hz ... Extremely low frequency (ELF)"[dead link]
- 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. 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."
- "Electromagnetic fields and public health". Fact Sheet No. 322, June 2007. World Health Organization, Accessed 7 February 2010.
- "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
- "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.
- 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
- Cleary, Stephen F. "Electromagnetic Field: A Danger?". The New Book of Knowledge - Medicine And Health. 1990. 164-74. ISBN 0-7172-8244-9.
- "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 and Newly Identified Health Risks-SCENIHR (January 2009). Health Effects of Exposure to EMF. Brussels: Directorate General for Health&Consumers; European Commission. pp. 4–5. Retrieved 2010-04-27.
- Extremely Low Frequency Fields Environmental Health Criteria Monograph No.238, chapter 5, page 121, WHO
- Non-ionizing radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC. (Non-Ionizing Radiation)
- 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
- Longitudinal Electric Waves