ultralowfrequency

Ultralow frequency (ULF) is a term used across disciplines to describe phenomena at the low end of the electromagnetic or acoustic spectrum. In radio engineering, ultralow frequency often refers to very low frequencies used for long-range signaling and geophysical studies, typically spanning from a few hertz to a few kilohertz, depending on convention. In geophysics and space physics, ULF describes natural fluctuations of electric and magnetic fields in the sub-Hertz to tens of Hz range, including phenomena such as Schumann resonances near 7.8 Hz that occur in the Earth–ionosphere cavity and various magnetohydrodynamic waves in the magnetosphere.

Natural sources include lightning, which emits broadband ULF energy, and geomagnetic activity driven by solar wind. Artificial sources have included submarine communication systems that use ultra-low frequencies because of their penetration through seawater, though such systems require large antennas and substantial power. Geophysical exploration and monitoring also employ controlled sources to excite ULF waves for subsurface imaging and to study crustal and atmospheric processes.

In acoustics, ultralow-frequency can refer to very low-frequency sounds, typically below 20 Hz, produced by natural processes such as earthquakes, volcanic activity, and ocean–atmosphere interactions, as well as by human activities. Measurement of ULF signals relies on long-duration observations and sensitive sensors; electromagnetic applications use magnetometers and electric-field probes, while acoustic applications rely on pressure sensors. The long wavelengths involved constrain spatial resolution and influence propagation through terrain, water, air, and the ionosphere.