Frequency range | 0.3 to 3kHz |
|---|---|
Wavelength range | 1,000 to 100km |
| Radio bands | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ITU | ||||||||||||
| ||||||||||||
| EU / NATO / US ECM | ||||||||||||
| IEEE | ||||||||||||
| Other TV and radio | ||||||||||||
Ultra low frequency (ULF) is theITU designation[1][2] for thefrequency range of electromagnetic waves between 300hertz and 3 kilohertz, corresponding to wavelengths between 1,000 and 100 km. Inmagnetosphere science andseismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz,[3] 1 mHz to 1 Hz,[4] and 10 mHz to 10 Hz.[5]
Many types of waves in the ULF frequency band can be observed in themagnetosphere 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 theAlfvén velocity that depends on the ambient magnetic field and plasma mass density.
This band is used for communications inmines, as it can penetrate the earth.[6]
Some monitoring stations have reported thatearthquakes are sometimes preceded by a spike in ULF activity. A remarkable example of this phenomena was believed to have occurred before the1989 Loma Prieta earthquake inCalifornia based on data from a nearby sensor array.[7] 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.[8]
On December 9, 2010, geoscientists announced that theDEMETER satellite observed a dramatic increase in ULF radio waves overHaiti in the month before the magnitude 7.0Mw2010 earthquake, and a gradual ebbing of the ULF waves in the month after the quake.[9][10] 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.
Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used inWorld War I. This technology was explored for possible use in the mining industry from the 1920s.[11][12]
ULF has been used by the military forsecure communications through the ground. Publications associated withNATO'sAGARD from the 1960s detailed many such systems,[13] although it is possible that the published papers left unmentioned some information about what actually was developedsecretly for defense purposes.
Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spacedelectrode pairs hammered into the soil. At the receiving end, the signal is detected as a weakelectric current between a further pair of electrodes. Using weak signal reception methods with PC-basedDSP 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.[citation needed]
Radio spectrum (ITU) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||
| |
| Gamma rays | |
| X-rays | |
| Ultraviolet | |
| Visible (optical) | |
| Infrared | |
| Microwaves | |
| Radio | |
| Wavelength types | |
