Schumann resonances

This global electromagnetic resonance phenomenon is named after physicist Winfried Otto Schumann who predicted it mathematically in 1952. Schumann resonance occurs because the space between the surface of the Earth and the conductive ionosphere acts as a waveguide. The limited dimensions of the Earth cause this waveguide to act as a resonant cavity for electromagnetic waves in the ELF band. The cavity is naturally excited by energy from lightning strikes. Schumann resonance modes are observed in the power spectra of the natural electromagnetic background noise, as separate peaks at extremely low frequencies (ELF) around 7.8, 14.3, 20.8, 27.3 and 33.8 Hz.

The fundamental mode of the Schumann resonance is a standing wave in the Earth-ionosphere cavity with a wavelength equal to the circumference of the Earth. This lowest-frequency (and highest-intensity) mode of the Schumann resonance occurs at a frequency of approximately 7.8 Hz. Further resonance modes appear at approximately 6.5 Hz intervals, a characteristic attributed to the atmosphere's spherical geometry. The peaks exhibit a spectral width of approximately 20% on account of the damping of the respective modes in the dissipative cavity. The eighth overtone lies at approximately 59.9 Hz.

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And here how to capture “exotic” signals in the range of ELF – SLF – ULF – VLF frequencies. Connecting an antenna to a PC with a Sound Card and Software like CiaoRadio, it is possible to receive, analyze, and demodulate any ELF – SLF – ULF – VLF signal up to 24 kHz.

Or intercept submarines communications! USA and Russia transmit in the ELF band generating a slow binary code. Frequencies are 76 Hz for the U.S. system and 82 Hz in the Russian system; the E.R.P. (Effective Radiated Power) is small, probably a few Watts, but signals are receivable around the world. This special kind of reception requires low-pass filters to avoid overloading the input with spherics. All spectrograms in this section are received with the Marconi T antenna and horizontal loop (2100 sq.m).