The Schumann resonances are naturally occuring electromagnetic waves, and are the principal component of natural electromagnetic radiation in the range of 7Hz to 50Hz. They have been shown to have relationships with both lightning strikes and the human body clock synchronisation.
At The Low End of the Spectrum
The research and development leading to VHF (Very High Frequency) and UHF (Ultra High Frequency) radio communications in the 1940s and 1950s led to scientific interest in the opposite end of the spectrum, ULF (Ultra Low Frequency) and ELF (Extremely Low Frequency). These exceptionally long waves were of scientific interest primarily because their wavelengths are comparable to the circumference of the Earth; also, because using such low frequencies makes possible communication with targets below the Earth's surface, such as submarines.
In a paper published in 1952, W. O. Schumann predicted naturally occurring resonances at extremely low frequencies1. The reasoning for this proposal is fairly straightforward, and is derived from the electrical properties of the Earth and its atmosphere.
The Earth's ionosphere is an electrical conductor, as is the Earth itself; between these, the lower atmosphere forms a resonating cavity. This resonator has a theoretical fundamental frequency, which depends on the speed of the wave (approximately 300,000 km/s for radio waves), and the circumference of the Earth (approximately 40,000 km)2:
f = v = 300 000 km/s = 7.5Hz
λ 40 000 km
Standing waves were theorised to exist at this frequency, and at other multiples of the base frequency. Such resonances were detected experimentally by Schumann and H. L. König in 19523. The peak is at around 7.8Hz, and several other waves are detectable at around 14, 20, 26, 33, 39, and 45 Hz, all with slight daily variation.
Research on these resonances continued in later years; ELF and VLF waves' interaction with human subjects was experimentally shown in the 1970s by König4, and a correlation between cloud-to-ground lightning and disturbances in the Schumann resonances was shown in the 1980s by Sentman5. Detection of the waves and research into their relationship to lightning continues6.
Studies performed by Wever7 showed that a complete absence of electric and magnetic fields led to a total desynchronisation of the human body clock to the outside world. Introducing an artificial electric field at around 10Hz resynchronised the circadian rhythms, suggesting that the body's natural rhythms are tied to the Schumann resonance.
1: W. O. Schumann, "Uber die Strahlungslosen Eigenschwingungen einer leitended Kügel die von Liftschicht unde einer Ionsphärenhülle umgeben ist", Zeitschrift für Naturforschung, vol. A, no. 7, pp. 149-154, 1952
2: P. V. Bliokh, A. P. Nicholaenko, and Yu. F. Fillipov, Schumann Resonances in the Earth-Ionosphere Cavity, ch. 1, pp. 2-3, The Institution of Electrical Engineers, London, 1980
3: W. O. Schumann and H. L. König, "Uber die beobachtung von atmospherics bei geringsten frequenzen", Naturwissenschaften, no 41, p. 183, 1954
4: H. L. König, "Behavioural changes in human subjects associated with ELF electric fields", in ELF and VLF Electromagnetic Field Effects (M. A. Persinger, ed.), Plenum Press, New York, 1974
5: Davis D. Sentman, "Schumann Resonances", in CRC Handbook of Atmospheric Electrodynamics (Hans Volland, ed.), CRC Press, Boca Raton, 1985
6: D. D. Sentman and B. J. Fraser, "Simultaneous observations of Schumann resonances in California and Australia: Evidence for intensity modulation by the local height of the D region", Journal of Geophysical Research, vol. 96, pp. 15973-15984, September 1991
7: R. Wever, "ELF effects on Human Circadian Rhythms", in ELF and VLF Electromagnetic Field Effects (M. A. Persinger, ed.), pp. 101-144, Plenum Press, New York, 1974