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Bioelectromagnetism
amplitude over a wide frequency band. Ground lightning with a duration of few microseconds can gen
erate global-scale, alternating-current electric felds.
Te atmospheric contains a wide variety of ions with diference in size and charge. Tese ions tend
to attach to aerosols. Tey originate from natural sources such as cosmic rays, radioactivity, splashing
water, and dust storms and anthropogenic sources such as high voltage power lines. Corona ions are
produced when the voltage is high enough to cause corona breakdown around cable.
3.3.2 Electromagnetic Fields and Resonance
In 1952, Schumann theorized that the space between the earth and ionosphere forms a spherical capaci
tor. Assuming the ionosphere to be a perfect refector, and based on the circumference of the earth-
ionosphere cavity and the velocity of light, he predicted that its frst mode of resonant frequency is 10 Hz
(Schumann 1952). Tis resonance phenomenon is now called Schumann resonance (SR). In 1954, König
frst reported the measurement of the resonance phenomena (König, 1977). Te circumference of this
cavity resonator is approximately equal to the wavelength of 7.8 Hz electromagnetic waves. More rigor
ously, as the electrical conductivity of ionosphere’s boundary layer are fnite, the fundamental resonant
frequency is about 7.8 Hz. Te harmonic resonant frequency can be obtained from following equation.
f = 7.8
Tis formula predicts a fundamental mode resonant frequency of f1 = 7.8 Hz and harmonic resonant
frequency, n= 1, 2, 3… Power spectrum of measured electric felds of natural signals has been described
(König et al., 1981). Te resonances can be clearly seen in the measurement of the power spectrum as
shown in Figure 3.4 (König et al., 1981; Toomey and Polk, 1970). Tere is a fundamental resonant fre
quency at about 8 Hz, and harmonic resonant frequencies are observed around 14, 20, and 26 Hz.
n n +
(
1)
2
Hz
FIGURE 3.4 Power spectra of the naturally generated electric feld in the ELF ranges (SR). (From König et al.,
1981.)