Extremely low frequency (ELF) is the ITU designation for electromagnetic radiation (radio waves) with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, an alternative definition is usually given, from 3 Hz to 3 kHz.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. In the related magnetosphere science, the lower-frequency electromagnetic oscillations (pulsations which occur below ~3 kHz) 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 by 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 waves have been used in only 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. Those transmitters consist of huge grounded wire antennas (ground dipoles) that are long, and are driven by megawatts of power. The United States, Russia, India, and China are the only countries that are known to have constructed these ELF communication facilities. at the Federation of American Scientists website The U.S. facilities were used between 1985 and 2004, but are now decommissioned.
Alternative definitions ELF is a subradio frequency.NASA.gov, page 8. ">0 to 300 Hz ... Extremely low frequency (ELF)". . 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.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. 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)"."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."
Propagation
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 curvature of the world. ELF and VLF waves propagate long distances via a ground–ionosphere waveguide mechanism. Earth is surrounded by a layer of charged particles (ions and electrons) in the atmosphere at an altitude of about 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 transverse electromagnetic mode (TEM) in vertical polarization, with the electric field vertical and the magnetic field horizontal. ELF waves have extremely low attenuation of 1–2 dB per ,Barr, et al (2000) ELF and VLF radio waves (), p. 1695, 1696 (fig. 3). 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 world several times before decaying to negligible amplitude, and thus waves that are radiated from sources in opposite directions circumnavigating the world on a great circle path interfere with each other.Barr, et al. (2000) ELF and VLF radio waves (), p. 1700–1701. At certain frequencies, these oppositely directed waves are in phase and add (reinforce), causing standing waves. In other words, the closed spherical ground–ionosphere cavity acts as a huge cavity resonator, enhancing ELF radiation at its resonant frequencies. These are called Schumann resonances, after the German physicist Winfried Otto Schumann, who predicted them in 1952, and were detected in the 1950s. Modelling the ground–ionosphere cavity with perfectly conducting walls, Schumann calculated that the resonances should occur at frequencies of
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 Earth, and higher harmonics occur at 14.1, 20.3, 26.4, and 32.4 Hz, etc. Lightning strikes excite these resonances, causing the ground–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.
Submarine communications
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. 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. The United States Navy in 1982 built the first ELF submarine communications facility, two coupled ELF transmitters at Clam Lake, Wisconsin, and Republic, Michigan."U.S. Navy: Vision...Presence...Power" (). SENSORS – Subsurface Sensors. US Navy. Accessed 7 February 2010. They were shut down in 2004. The Russian Navy operates an ELF transmitter called ZEVS (Zeus) at Murmansk on the Kola Peninsula.ZEVS, the Russian 82 Hz ELF transmitter. The Indian Navy has an ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant-class and Akula-class submarines.
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 that is 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. For example, a 3 Hz signal has a wavelength that is equal to the distance that 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 per second divided by 50–85 Hz, which equals around long. This is comparable to Earth's diameter of around . Because of this huge size requirement, to transmit internationally using ELF frequencies, the world 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 power lines, so-called ground dipoles, as leads. These leads were in multiple strands ranging from long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.
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 onto, or onto part of, the "pig", which is the cleaning device that is 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 that are located outside of the pipe.Stéphane Sainson, Inspection en ligne des pipelines. Principes et méthodes . Ed. Lavoisier. 2007. . 332 p. 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 replay them at higher speeds to more easily observe natural low-frequency fluctuations in Earth's electromagnetic field. Increasing the playback speed increases the pitch, bringing the tone into the audio frequency range.
Since the 2000s, very low frequencies have been used successfully at sea for oil geophysical prospecting.Stéphane Sainson, Electromagnetic seabed logging, A new tool for geoscientists. Ed. Springer, 2016
Natural sources Naturally occurring ELF waves are present on Earth, resonating in the region between the ionosphere and the surface, seen in lightning strikes that make electrons in the atmosphere oscillate. Republished as "Casini - Unlocking Saturn's Secrets - Titan's mysterious radio wave ". 22 November 2007. NASA. Accessed 7 February 2010. Although signals that are 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.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. Also, the fundamental mode of the ground–ionosphere cavity has the wavelength equal to the circumference of 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.