What the Schumann resonance is — definition and physical basis

The Schumann resonance is a set of global electromagnetic resonances in the cavity between the conductive surface of the Earth and the conductive, lossy lower ionosphere. These are therefore standing electromagnetic waves in the extremely low-frequency band that "circle" the entire planet.

The excitation is supplied by global lightning activity: every second, on the order of dozens of lightning strikes hit the Earth, and each discharge radiates a broad spectrum of electromagnetic energy. The frequencies that match the dimensions of the Earth-ionosphere cavity add up constructively and create stable resonance peaks; other frequencies, by contrast, are rapidly damped. The existence of the phenomenon was predicted theoretically by Winfried Otto Schumann in 1952 Schumann 1952.

Because the ionosphere is a lossy conductor, the cavity is not an ideal resonator — as the waves propagate they lose energy, which broadens and shifts the observed peaks. The SR is therefore a global "natural electromagnetic phenomenon" powered by lightning, not an artificial or living signal. It is measured with sensitive magnetic and electric antennas at remote, electromagnetically quiet locations. The SR also serves as a global "thermometer" of tropical storm activity Price 2016Sentman 1995.