Extremely low frequency - AbsoluteAstronomy.com

Extremely low frequency (ELF) is a term used to describe radiation frequencies from 3 to 300 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere

Magnetosphere

A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF

Ultra low frequency

Ultra-low frequency is the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In magnetosphere science and seismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to 10 Hz...

range, which is thus also defined differently from the ITU Radio Bands.

Description

ELF is a subradio frequency. Some medical peer review

Peer review

Peer review is a process of self-regulation by a profession or a process of evaluation involving qualified individuals within the relevant field. Peer review methods are employed to maintain standards, improve performance and provide credibility...

ed 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

Who may refer to:* Who , an English-language pronoun* who , a Unix command* Who?, one of the Five Ws in journalism- Art and entertainment :* Who? , a 1958 novel by Algis Budrys...

(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."

Military communications

The United States Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

utilized extremely low frequencies (ELFs) as radio band and radio communications. The Submarine Integrated Antena System (SIAS) was a research and development effort to communicate with submerged submarines

Communication with submarines

Communication with submarines is difficult because radio waves do not travel well through thick electrical conductors like salt water.The obvious solution is to surface and raise an antenna above the water, then use ordinary radio transmissions. Early submarines had to frequently surface anyway for...

. The Soviet

Soviet Navy

The Soviet Navy was the naval arm of the Soviet Armed Forces. Often referred to as the Red Fleet, the Soviet Navy would have played an instrumental role in a Warsaw Pact war with NATO, where it would have attempted to prevent naval convoys from bringing reinforcements across the Atlantic Ocean...

/Russian Navy also utilized ELFs for submarine communications system, ZEVS.

Explanation

Because of the electrical conductivity of seawater

Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

, submarines are shielded from most electromagnetic

Electromagnetic radiation

Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

communication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

s. 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. This is because the antenna must be at least a substantial fraction of the size (in at least one dimension) of the wavelength of the frequency of the EM waves. Simply put, a 1 Hz

Hertz

(cycle per second) signal would have a wavelength equal to the distance EM waves travel through a given medium in 1 second. For ELF, this is very slightly slower than the speed of light in a vacuum

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

. As used in military applications, the wavelength is ~299,792 km (~186,282 mi) per second divided by 50–85 Hz, which equals around 3527 kilometre long; by comparison, Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's diameter is around 12742 km (7,917.5 mi). Because of this huge size requirement and, to transmit internationally using ELF frequencies, the earth itself must be used as an antenna, with extremely long leads going into the ground. Various other means are taken to construct radio stations with substantially smaller sizes, such as electrical lengthening

Electrical lengthening

Electrical lengthening is the modification of an aerial which is shorter than a whole-number multiple of a quarter of the radiated wavelength, by means of a suitable electronic device, without changing the physical length of the aerial, in such a way that it corresponds electrically to the next...

.

The US maintained two sites, in the Chequamegon-Nicolet National Forest

Chequamegon-Nicolet National Forest

The Chequamegon-Nicolet National Forest is a 1,530,647 acre U.S. National Forest in northern Wisconsin in the United States. Much of the old growth forest in this region was destroyed by logging in the early part of the 20th century...

, Wisconsin

Wisconsin

Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...

and the Escanaba River State Forest

Escanaba River State Forest

Escanaba River State Forest is a state forest in the central Upper Peninsula of Michigan. It includes forested lands in Alger, Delta, Marquette, and Menominee Counties...

, Michigan

Michigan

Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

(originally named Project Sanguine

Project Sanguine

Project Sanguine was a project to research a system for communicating with submarines using extremely long radio waves.-References and external articles:* James R. Wait, Project Sanguine. Science, 1972....

, then downsized and rechristened Project ELF prior to construction), until they were dismantled, beginning in late September 2004. Both sites used long power line

Electric power transmission

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...

s, so-called ground dipole

Ground dipole

A Ground dipole, or Ground antenna, is a special type of antenna which consists of two electrodes positioned in the ground.Ground dipole antennas are usually used for transmitting and receiving on the ELF and VLF bands. The antenna consists of two electrodes, planted in the ground at some distance...

s, as leads. These leads were in multiple strands ranging from 22.5 to 45 km (14 to 28 ) long. Because of the inefficiency of this method, considerable amounts of electrical power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

were required to operate the system.

Ecological impact

There have been some concerns over the possible ecological

Ecology

Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

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

Herb Kohl

Herbert H. "Herb" Kohl is the senior U.S. Senator from Wisconsin and a member of the Democratic Party. He is also a philanthropist and the owner of the Milwaukee Bucks National Basketball Association team...

, Russ Feingold

Russ Feingold

Russell Dana "Russ" Feingold is an American politician from the U.S. state of Wisconsin. He served as a Democratic party member of the U.S. Senate from 1993 to 2011. From 1983 to 1993, Feingold was a Wisconsin State Senator representing the 27th District.He is a recipient of the John F...

and Congressman Dave Obey

Dave Obey

David Ross "Dave" Obey is the former U.S. Representative for , serving 21 consecutive terms from 1969 until 2011. The district includes much of the northwestern portion of the state, including Wausau and Superior...

called for its closure. Similar concerns have in the past been raised about electromagnetic radiation and health.

Other uses

Transmitters in the 20 Hz range are also found in pipeline inspection gauges, also known as "PIGs".

Some radio hams

Amateur radio

Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

record ELF (or even lower) signals from very large homemade antennas, and play them back at higher speeds

Pitch control

A variable speed pitch control is a control on an audio device such as a turntable, tape recorder, or CD player that allows the operator to deviate from a standard speed . The latter term "vari-speed" is more commonly used for tape decks, particularly in the UK...

to catch natural fluctuations in the Earth's electromagnetic field. Increasing the playback increases the pitch

Pitch (music)

Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

, so that it is brought into the audio frequency

Audio frequency

An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...

range.

Natural sources

Naturally occurring ELF waves are present on Earth, resonating in the region between ionosphere

Ionosphere

The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

and surface. They are initiated by lightning

Lightning

Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

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

Schumann resonance

The Schumann resonances are a set of spectrum peaks in the extremely low frequency portion of the Earth's electromagnetic field spectrum...

.

They have also been tentatively identified on Saturn's moon Titan

Titan (moon)

Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

. 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 1200 km (745.6 mi) but with an additional layer of charged particles at 63 km (39.1 mi). This splits Titan's atmosphere to some extent 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 magnetar

Magnetar

A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious high-energy electromagnetic radiation, particularly X-rays and gamma rays...

s. The pulsar in the Crab nebula

Crab Pulsar

The Crab Pulsar is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054...

radiates powers of this order at the frequency 30 hertz http://www.cv.nrao.edu/course/astr534/Pulsars.html. Radiation of this frequency is below the plasma frequency of the interstellar medium

Interstellar medium

In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

, thus this medium is opaque to it, and it cannot be observed from Earth.

Exposure

In electromagnetic therapy

Electromagnetic therapy

Electromagnetic therapy, is a form of alternative medicine which claims to treat disease by applying electromagnetic radiation or pulsed electromagnetic fields to the body....

and electromagnetic radiation and health research, electromagnetic spectrum

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

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.

To put this into context, it is important to understand that the "possibly carcinogenic" classification is also applied to coffee, gasoline engine exhaust and pickled vegetables, and is often used for agents that require further study. 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

Alzheimer's disease

Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...

Patents

Tanner, R. L., , "Extremely low-frequency antenna". 1965.
Hansell, Clarence W., , "Communication system by pulses through the Earth".
Altshuler, , ELF vertical dipole antenna suspended from aircraft

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
Longitudinal Electric Waves

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