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Bioelectromagnetism

FIGURE 3.2 Intensity of natural electric and magnetic felds in the frequency range 0–50 kHz. Circle: electric

feld (E) in V/m, Triangles: magnetic feld (B) in 10⁻⁹ T. (From Oehrl and König, 1968.)

consists primarily of electromagnetic waves in the ELF and VLF ranges. In ELF atmospherics, the

electromagnetic phenomena measured over the earth’s surface result from the excitation of the earth-

ionosphere cavity resonator by thunderstorms. Te earth’s surface and the lower ionosphere are good

conductors. Te air between them is an insulating layer. Tis forms a waveguide for electromagnetic

waves which depends on frequency. Tis waveguide forms earth-ionosphere cavity for low-frequency

electromagnetic waves.

When ELF electromagnetic felds are produced by lightning discharge, the electric feld causes electric

current fow in the atmosphere. Magnetic felds of the same frequency as the result of current fow are

produced. Te higher the frequency of the electromagnetic waves, the greater the attenuation and the

lesser the propagation range. As described by Oehrl and König (1968), the electric feld and the magnetic

feld intensity and frequency range 0–50 kHz measured at one point are inversely proportional in log-log

plot (Figure 3.2). Te amplitude of the electric feld is averaged 10⁻²V/m at 1Hz and 10⁻⁶ V/m at 3kHz. Tis

shows a relationship between electric and magnetic felds and frequency. Natural electric and magnetic

feld strengths decrease rapidly with increasing frequency. Te strength of ultra-low frequency electric

and magnetic felds over a broad frequency range is about 10⁻³−10⁻⁵ V/m and 10⁻¹²−10⁻¹⁴ T, respectively. Te

E/H ratio (≈370 Ω) corresponds to that of electromagnetic waves. Te E/H is the wave impedance.

Te observed wave forms vary with the distance—which can be from 50 to 15,000 km—from the light­

ning strike (Figure 3.3) (Al’pert and Fligel, 1995). When the distance is short, the waveform is a single

pulse. However, at distances greater than 1,000 km, the waveform approaches an oscillating form with

a defnite periodicity. Te changing of the waveform originates from the electromagnetic wave emitted

by lightning, which propagates by refecting between the ionosphere and the earth’s surface, which acts

as a perfect conductor. Tis phenomenon exhibits resonance at specifc frequencies. Te space between

the earth and ionosphere serves as a large waveguide for atmospherics. Of the signals propagating from

lightning discharges, low-frequency components have low attenuation and can circle the earth several

times. Standing waves develop from the excitation of the spherical, surface-cavity resonator between the

earth’s surface and the lower boundary of the ionosphere. Te fundamental frequency of this resonance

is near 7.5 Hz, which is determined by dividing the propagation speed of the electromagnetic wave with

light velocity (3 × 10⁵ km/s) by the diameter of the earth (4 × 10⁴km).

Te quasi-static (i.e., relatively constant) feld consists of a negatively charged earth and a positively

charged atmosphere. Te ground-level electric feld is about 0.1 kV/m during fair weather, but electric

felds above 100 kV/m have been observed during thunderstorms. Atmospherics exhibit considerable