Extremely low frequencies (ELF) and placement in the EM spectrum

The Schumann resonance falls within the band of extremely low frequencies (ELF, extremely low frequency), roughly 3-60 Hz. This is the very bottom edge of the electromagnetic spectrum, far below radio waves and many orders of magnitude below visible light.

In this band the wavelengths are enormous. At frequencies of a few to tens of hertz they reach tens of thousands of kilometers, comparable to the dimensions of the planet itself. This is precisely what enables the formation of global standing waves: the wave "fits" around the entire Earth. ELF waves moreover propagate with very little attenuation over immense distances, so a single strong lightning strike in the tropics is detectable on the other side of the world.

For comparison: ordinary AM broadcasting operates at hundreds of kHz, mobile networks at hundreds of MHz to a few GHz — these are frequencies six to nine orders of magnitude higher than the SR. The Schumann resonances are therefore not a "radio signal" in the usual sense, but very slow, planetary-scale oscillations of the electromagnetic field Price 2016Sentman 1995. Measurement in the ELF band therefore requires special antennas and sensitive receivers Tatsis 2024.