Ultra low frequency (ULF) is the

ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Union ...

designation for the

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

range of electromagnetic waves between 300

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

and 3 kilohertz, corresponding to wavelengths between 1,000 to 100 km. In

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior d ...

science and

seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, and 10 mHz to 10 Hz. Many types of waves in the ULF frequency band can be observed in the

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 mines, as it can penetrate the earth.

Earthquakes

Some monitoring stations have reported that

earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fro ...

are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the

1989 Loma Prieta earthquake The 1989 Loma Prieta earthquake occurred on California's Central Coast on October 17 at local time. The shock was centered in The Forest of Nisene Marks State Park in Santa Cruz County, approximately northeast of Santa Cruz on a section of t ...

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

, although a subsequent study indicates that this was little more than a sensor malfunction. On December 9, 2010, geoscientists announced that the

DEMETER In ancient Greek religion and Greek mythology, mythology, Demeter (; Attic Greek, Attic: ''Dēmḗtēr'' ; Doric Greek, Doric: ''Dāmā́tēr'') is the Twelve Olympians, Olympian goddess of the harvest and agriculture, presiding over crops, ...

satellite observed a dramatic increase in ULF radio waves over

Haiti Haiti (; ht, Ayiti ; French: ), officially the Republic of Haiti (); ) and formerly known as Hayti, is a country located on the island of Hispaniola in the Greater Antilles archipelago of the Caribbean Sea, east of Cuba and Jamaica, and s ...

in the month before the magnitude 7.0 M_w

2010 earthquake Earthquakes in 2010 resulted in nearly 165,000 fatalities. Most of these were due to the 2010 Haiti earthquake, which caused an estimated 160,000 deaths, making it the 11th deadliest earthquake in recorded history. Other deadly quakes occurred i ...

. 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

ULF has been used by the military for secure communications through the ground. NATO AGARD publications from the 1960s detailed many such systems, although it is possible that the published papers left a lot unsaid about what actually was developed secretly for defense purposes. Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

Radio amateurs An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators ha ...

and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...

pairs hammered into the soil. At the receiving end, the signal is detected as a weak

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

between a further pair of electrodes. Using weak signal reception methods with PC-based DSP filtering with extremely narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10100W and electrode spacing of around 1050m.

References

External articles

* Tomislav Stimac, "
Definition of frequency bands (VLF, ELF... etc.)
'". IK1QFK Home Page (vlf.it).

* Amateur Radio Below 10 kHz

* Review of Earth Mode Communications
1966 abstract about Earth Mode Comms by Ames, Frazier and Orange
* Radio communications within the Earth's crust
Abstract of article by Burrows written in 1963
{{DEFAULTSORT:Ultra Low Frequency Radio spectrum

Earthquakes

Earth mode communications

See also

References

External articles