Basic Theory
Lightning discharges are considered to be the primary natural source of Schumann resonance excitation; lightning channels behave like huge antennas that radiate electromagnetic energy at frequencies below about 100 kHz. These signals are very weak at large distances from the lightning source, but the Earth–ionosphere waveguide behaves like a resonator at ELF frequencies and amplifies the spectral signals from lightning at the resonance frequencies.
In an ideal cavity, the resonant frequency of the -th mode is determined by the Earth radius and the speed of light .
The real Earth–ionosphere waveguide is not a perfect electromagnetic resonant cavity. Losses due to finite ionosphere electrical conductivity lower the propagation speed of electromagnetic signals in the cavity, resulting in a resonance frequency that is lower than would be expected in an ideal case, and the observed peaks are wide. In addition, there are a number of horizontal asymmetries – day-night difference in the height of the ionosphere, latitudinal changes in the Earth magnetic field, sudden ionospheric disturbances, polar cap absorption, variation in the Earth radius of +/- 11km from equator to geographic poles, etc. that produce other effects in the Schumann resonance power spectra.
Read more about this topic: Schumann Resonances
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