Extremely Low Frequency
Extremely low frequency (ELF) waves are electromagnetic radiation (radio waves) with frequencies from 3 to 300 Hz, and corresponding wavelengths from 100,000 to 1000 kilometers. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU Radio Bands.
ELF radio waves are generated by lightning and natural disturbances in Earth's magnetic field, so they are a subject of research by atmospheric scientists. Because of the difficulty of building transmitters that can generate such long waves, ELF frequencies have been used in only a very few manmade communication systems. ELF waves can penetrate seawater to a depth of several hundred feet, so the US and Russian military have used ELF transmission facilities to communicate with their submerged submarines. The frequency of alternating current flowing in electric power grids, 50 or 60 Hz, falls within this frequency band, and power grids are an unintentional source of ELF radiation.
Read more about Extremely Low Frequency: Alternate Definitions, Military Communications, Other Uses, Natural Sources, Exposure, Patents
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