English: Typical spectrum of extremely low frequency (ELF) electromagnetic oscillations in the Earth's atmosphere, showing peaks caused by the Schumann resonances. The Schuman resonances are the resonant frequencies of the spherical cavity bounded by the Earth and the ionosphere; lighning strikes and other sferics cause the Earth-ionosphere cavity to "ring" like a bell at these frequencies, causing peaks in the noise spectrum. The sharp power peak at 50 Hz is caused by radiation from global electric power grids. The rise of the noise at low frequencies (left side) is radio noise caused by slow processes in the Earth's magnetosphere.