Ultra low frequency (ULF) is the name given by the ITU for electromagnetic waves with frequencies between 300 hertz and 3 kilohertz. These frequencies correspond to wavelengths between 1,000 and 100 kilometers. In magnetosphere science and seismology, other definitions are often used, such as frequencies ranging from 1 mHz to 100 Hz, 1 mHz to 1 Hz, and 10 mHz to 10 Hz.
Many types of waves within the ULF range can be observed in the magnetosphere and on Earth's surface. These waves are important for understanding physical processes in the plasma environment near Earth. The speed of ULF waves is often linked to the Alfvén velocity, which depends on the strength of the magnetic field and the density of the plasma.
This frequency band is used for communication in mines because the waves can pass through Earth's layers.
Earthquakes
Some monitoring stations have found that earthquakes may sometimes be followed by a sudden rise in ULF activity. An example of this was thought to happen before the 1989 Loma Prieta earthquake in California, based on data from a group of nearby sensors. However, a later study suggested that the increase in ULF activity was likely caused by a problem with the sensors, as other sensors in different areas did not detect the same change.
On December 9, 2010, scientists reported that the DEMETER satellite recorded a large increase in ULF radio waves over Haiti in the month before the magnitude 7.0 Mw 2010 earthquake. The ULF waves then gradually decreased in the month after the earthquake. Researchers are working to understand this connection to determine if it can help create an early warning system for future earthquakes.
Earth mode communications
Earth-Mode communications is a method of sending messages through the ground using conduction fields. This technology was first used during World War I. Scientists began studying its use for mining operations in the 1920s.
Ultra-Low Frequency (ULF) signals have been used by the military for secure underground communication. Reports from NATO's AGARD organization in the 1960s described several systems. However, some details about secret defense projects may not have been fully shared in these reports.
Radio hobbyists and electronics enthusiasts have used Earth-Mode for short-distance communication. They connect amplifiers that boost sound signals to pairs of electrodes driven into the soil. At the receiving end, a weak electric current between another pair of electrodes detects the signal. Using special computer programs that filter signals with very narrow ranges, messages can be received up to a few kilometers away. This requires a transmitting power of 10–100 watts and electrode spacing of about 10–50 meters.
External articles
- Tomislav Stimac, "Definition of Frequency Bands (VLF, ELF, etc.)". IK1QFK Home Page (vlf.it).
- NASA Live Streaming ELF to VLF Receiver
- Amateur Radio Below 10 kHz: "G3XBM's Page on Earth Mode Communication"
- Review of Earth Mode Communications: "1966 Abstract About Earth Mode Communications by Ames, Frazier, and Orange"
- Radio Communications Within the Earth's Crust: "Abstract of an Article by Burrows from 1963"
- OEAW, " [1] ". ULF Waves – According to the Geophysical Research Community, Homepage of OEAW, Austrian Academy of Sciences, Austria.
ELF: 3 Hz to 30 Hz (100 Mm to 10 Mm)
SLF: 30 Hz to 300 Hz (10 Mm to 1 Mm)
ULF: 300 Hz to 3 kHz (1 Mm to 100 km)
VLF: 3 kHz to 30 kHz (100 km to 10 km)
LF: 30 kHz to 300 kHz (10 km to 1 km)
MF: 300 kHz to 3 MHz (1 km to 100 m)
HF: 3 MHz to 30 MHz (100 m to 10 m)
VHF: 30 MHz to 300 MHz (10 m to 1 m)
UHF: 300 MHz to 3 GHz (1 m to 100 mm)
SHF: 3 GHz to 30 GHz (100 mm to 10 mm)
EHF: 30 GHz to 300 GHz (10 mm to 1 mm)
THF: 300 GHz to 3 THz (1 mm to 0.1 mm)