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.
Read more about Ultra-low Frequency: Earthquakes, Earth Mode Communications
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