View source for Ultra low frequency

{{short description|Range 300-3000 Hz of the electromagnetic spectrum}}
{{MWband
| freq = 0.3 to 3 [[Kilohertz|kHz]]
| wave = 1,000 to 100 [[Meter|km]]
}}
[[File:Ambrose Channel pilot cable in action.PNG|thumb|Listening to 500 Hz signal of [[Ambrose Channel pilot cable]] in 1920]]
'''Ultra low frequency''' ('''ULF''') is the [[International Telecommunication Union|ITU]] designation<ref name="1037B">{{cite book | title = US Federal Standard 1037B: Telecommunications, Glossary of Telecommunications Terms | publisher = Office of Technology Standards, General Services Administration | date = 3 June 1991 | pages = S-18 | url = https://books.google.com/books?id=zYLXU4fkD34C&pg=RA15-PA18}}</ref><ref name="itu-2015-acts">{{cite conference |date=2015 |title=Final Acts WRC-15 |url=https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.12-2015-PDF-E.pdf |conference=World Radiocommunication Conference |location=Geneva, Switzerland |publisher=International Telecommunications Union |pages = 4|access-date=2025-01-12}}</ref> for the [[frequency]] range of electromagnetic waves between 300 [[hertz]] and 3 kilohertz, corresponding to wavelengths between 1,000 and 100&nbsp;km. In [[magnetosphere]] science and [[seismology]], alternative definitions are usually given, including ranges from 1 mHz to 100&nbsp;Hz,<ref>V. A. Pilipenko, "ULF waves on the ground and in space", [[Journal of Atmospheric and Terrestrial Physics]], Volume 52, Issue 12, December 1990, pp. 1193–1209, {{ISSN|0021-9169}}, {{doi|10.1016/0021-9169(90)90087-4}}.</ref> 1 mHz to 1&nbsp;Hz,<ref>T. Bösinger and S. L. Shalimov, "On ULF Signatures of Lightning Discharges", [[Space Science Reviews]], Volume 137, Issue 1, pp. 521–532, June 2008, {{doi|10.1007/s11214-008-9333-4}}.</ref> and 10 mHz to 10&nbsp;Hz.<ref>O. Molchanov, A. Schekotov, E. Fedorov, G. Belyaev, and E. Gordeev, "[http://www.nat-hazards-earth-syst-sci.net/3/203/2003/nhess-3-203-2003.pdf Preseismic ULF electromagnetic effect from observation at Kamchatka]", ''Natural Hazards and Earth System'' Sciences, Volume 3, pp. 203–209, 2003</ref>

Many types of waves in the ULF frequency band can be observed in the [[magnetosphere]] and on the ground.  These waves represent important physical processes in the near-Earth plasma environment.  The speed of the ULF waves is often associated with the [[Alfvén velocity]] that depends on the ambient magnetic field and plasma mass density.

This band is used for communications in [[Mining|mine]]s, as it can penetrate the earth.<ref>[http://www.weather.nps.navy.mil/~psguest/EMEO_online/module3/module_3_1.html HF and Lower Frequency Radiation - Introduction<!-- Bot generated title -->] {{Webarchive|url=https://web.archive.org/web/20051109130358/http://www.weather.nps.navy.mil/~psguest/EMEO_online/module3/module_3_1.html |date=2005-11-09 }}</ref>

==Earthquakes==
Some monitoring stations have reported that [[earthquakes]] are sometimes preceded by a spike in ULF activity.  A remarkable example of this phenomena was believed to have occurred before the [[1989 Loma Prieta earthquake]] in [[California]] based on data from a nearby sensor array.<ref>{{Cite journal |last=Fraser-Smith |first=Antony C. |last2=Bernardi |first2=A. |last3=McGill |first3=P. R. |last4=Ladd |first4=M. E. |last5=Helliwell |first5=R. A. |last6=Villard, Jr. |first6=O. G. |date=August 1990 |title=Low-Frequency Magnetic Field Measurements Near the Epicenter of the M<sub>s</sub> 7.1 Loma Prieta Earthquake |url=http://ee.stanford.edu/~acfs/LomaPrietaPaper.pdf |journal=[[Geophysical Research Letters]] |volume=17 |issue=9 |pages=1465–1468 |bibcode=1990GeoRL..17.1465F |doi=10.1029/GL017i009p01465 |issn=0094-8276 |oclc=1795290 |access-date=December 18, 2010}}</ref> However, a subsequent study argued this 1989 event was little more than a sensor malfunction, as sensors from other locations did not identify the ULF spike reported from near the quake's epicenter.<ref>{{Cite journal |last=Thomas |first=J. N. |last2=Love |first2=J. J. |last3=Johnston |first3=M. J. S. |date=April 2009 |title=On the reported magnetic precursor of the 1989 Loma Prieta earthquake |journal=[[Physics of the Earth and Planetary Interiors]] |volume=173 |issue=3–4 |pages=207–215 |bibcode=2009PEPI..173..207T |doi=10.1016/j.pepi.2008.11.014}}</ref> 

On December 9, 2010, geoscientists announced that the [[Demeter (satellite)|DEMETER]] satellite observed a dramatic increase in ULF radio waves over [[Haiti]] in the month before the magnitude 7.0 [[Moment magnitude scale|M<sub>w</sub>]] [[2010 Haiti earthquake|2010 earthquake]], and a gradual ebbing of the ULF waves in the month after the quake.<ref>Athanasiou, M. A., Anagnostopoulos, G. C., Iliopoulos, A. C., Pavlos, G. P., and David, C. N.: Enhanced ULF radiation observed by DEMETER two months around the strong 2010 Haiti earthquake, Nat. Hazards Earth Syst. Sci., 11, 1091–1098, https://doi.org/10.5194/nhess-11-1091-2011, 2011.</ref><ref>{{Cite web |last=KentuckyFC <!-- blogger's legitimate nom de plume --> |date=December 9, 2010 |title=Spacecraft Saw ULF Radio Emissions over Haiti before January Quake |url=https://www.technologyreview.com/2010/12/09/89531/spacecraft-saw-ulf-radio-emissions-over-haiti-before-january-quake/ |access-date=December 18, 2010 |publisher=[[MIT Technology Review]] |location=[[Cambridge, Massachusetts|Cambridge]], [[Massachusetts]]}}</ref>  Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes.

==Earth mode communications==
Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in [[World War I]]. This technology was explored for possible use in the mining industry from the 1920s.<ref>C. L. Colburn C. M. Bouton H. B. Freaman RI 2407 Experiments in underground signalling [sic] with radio sets. The National Institute for Occupational Safety and Health (NIOSH), 1922.</ref><ref>Coggeshall, E. J., E. W. Felegy, and L. H. Harrison. Some Studies on Emergency Mine Communications. BuMines RI 4135, 1948, 44 pp. </ref> 

ULF has been used by the military for [[secure communications]] through the ground. Publications associated with [[NATO]]'s [[AGARD]] from the 1960s detailed many such systems,<ref>C. K. H. Tsao and J. T. deBettencourt, "Subsurface radio propagation experiments," in Radio Science, vol. 3, no. 11, pp. 1039-1044, Nov. 1968, doi: 10.1002/rds19683111039</ref> although it is possible that the published papers left unmentioned some information about what actually was developed [[classified information|secretly]] for defense purposes. 

[[Radio amateurs]] and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced [[electrode]] pairs hammered into the soil. At the receiving end, the signal is detected as a weak [[electric current]] between a further pair of electrodes. Using weak signal reception methods with PC-based [[Digital signal processing|DSP]] filtering with extremely narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10{{ndash}}100{{nbsp}}W and electrode spacing of around 10{{ndash}}50{{nbsp}}m.{{Citation needed|date=September 2020|reason=Many facts are presented but none cited.}}

==See also==
* [[Extremely low frequency]]
* [[Earth's field NMR]]
* [[Valery Troitskaya]]
* [[Through-the-earth mine communications]]
* [[Voice frequency]]

==References==
<references/>

==External articles==
* Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''".  IK1QFK Home Page (vlf.it).
* [http://spaceweather.com/glossary/inspire.html NASA live streaming ELF -> VLF Receiver]
* Amateur Radio Below 10&nbsp;kHz "[http://homepage.ntlworld.com/lapthorn/earthmode.htm G3XBM's page on Earth Mode Communication]"
* Review of Earth Mode Communications "[https://web.archive.org/web/20070526023226/http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0640319 1966 abstract about Earth Mode Comms by Ames, Frazier and Orange]"
* Radio communications within the Earth's crust "[https://ieeexplore.ieee.org/document/1138037/;jsessionid=FA32016E43E347E8674D89296EE463A0?arnumber=1138037 Abstract of article by Burrows written in 1963]"
* OEAW, "''[https://www.oeaw.ac.at/en/iwf/research/research-groups/space-plasma-physics/ulf-waves#:~:text=Ultra%2DLow%20Frequency%20(ULF),planets%20and%20in%20interplanetary%20space.]'''". ULF Waves -according to geophysical research community Homepage of OEAW, Austrian academy of science, Austria.

{{radio_spectrum}}

{{EMSpectrum}}

{{DEFAULTSORT:Ultra Low Frequency}}
[[Category:Radio spectrum]]