Ultra low frequency
||0.3 to 3 kHz
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.
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 Mw 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 10-100W and electrode spacing of around 10-50m.
- ^ V.A. Pilipenko, "ULF waves on the ground and in space", Journal of Atmospheric and Terrestrial Physics, Volume 52, Issue 12, December 1990, Pages 1193-1209, ISSN 0021-9169, DOI:10.1016/0021-9169(90)90087-4.
- ^ T. Bösinger and S. L. Shalimov, "On ULF Signatures of Lightning Discharges", Space Science Reviews, Volume 137, Issue 1, Pages 521-532, June 2008, DOI:10.1007/s11214-008-9333-4.
- ^ O. Molchanov, A. Schekotov, E. Fedorov, G. Belyaev, and E. Gordeev, "Preseismic ULF electromagnetic effect from observation at Kamchatka", Natural Hazards and Earth System Sciences, Volume 3, Pages 203-209, 2003
- ^ HF and Lower Frequency Radiation - Introduction
- ^ Fraser-Smith, Antony C.; Bernardi, A.; McGill, P. R.; Ladd, M. E.; Helliwell, R. A.; Villard, Jr., O. G. (August 1990). "Low-Frequency Magnetic Field Measurements Near the Epicenter of the Ms 7.1 Loma Prieta Earthquake" (PDF). Geophysical Research Letters (Washington, D.C.: American Geophysical Union) 17 (9): 1465–1468. ISSN 0094-8276. OCLC 1795290. http://ee.stanford.edu/~acfs/LomaPrietaPaper.pdf. Retrieved December 18, 2010.
- ^ KentuckyFC (December 9, 2010). "Spacecraft Saw ULF Radio Emissions over Haiti before January Quake". Physics arXiv Blog. Cambridge, Massachusetts: TechnologyReview.com. http://www.technologyreview.com/blog/arxiv/26114/. Retrieved December 18, 2010.