The Earth-ionosphere cavity as a global resonator

The Earth-ionosphere cavity is the thin spherical shell between the conductive Earth's surface and the lower boundary of the ionosphere. This shell acts as a closed resonator for electromagnetic waves of extremely low frequencies.

Both "walls" of the cavity are electrically conductive: the solid and ocean surface of the Earth from below, and the ionized upper atmosphere (ionosphere) from above. Between them lies relatively non-conductive air. A wave of suitable wavelength reflects between these walls and, after travelling around the planet's circumference, returns to itself in phase, so resonance arises. The lower boundary of the effective resonator lies on the order of tens of kilometers above the surface, and its height varies with the time of day and solar irradiation.

The key word is "lossy": the ionosphere is not a perfect conductor, so part of the energy is absorbed at each reflection. The resonator is therefore strongly damped, its resonances are broad and somewhat shifted relative to the ideal case. It is continuously excited by global lightning activity, which feeds energy into a broad band of frequencies Sentman 1995Price 2016. The geometry and wall conductivity of the cavity directly determine the frequencies at which resonance occurs Schumann 1952.