Ultra-low frequency

Ultra-low frequency Frequency range 0.3 to 3 kHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11 12 ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF THF NATO Radio bands A B C D E F G H I J K L M IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

Ultra-low frequency (ULF) 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. Frequencies above 3 Hz in atmosphere science are usually assigned to the ELF range.

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 Alfven 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 are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the 1989 Loma Prieta earthquake in California. On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 M_w 2010 earthquake. 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 one suspects the contents of 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 WWI. 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 two further pairs 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 10-100 W and electrode spacing of around 10-50 m.

See also

• Earth's field NMR
• Through the earth mine communications
• Voice frequency

External articles

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
• NASA live streaming ELF -> VLF Receiver
• Amateur Radio Below 10 kHz "G3XBM's page on Earth Mode Communication"
• 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"