Extremely Low Frequency (ELF) Radio Wave Propagation: A review

The Extremely Low Frequencies (ELF) extend from 3 Hz to 3 kHz. This formal limit corresponds to real physical phenomena in subionospheric radio propagation: the Schumann Resonance (SR) observed in the band between 4 Hz and 40 Hz and the transverse resonance having the basic frequency about 1.7 kHz. When speaking about ELF radio waves, we usually have in mind the subionospheric propagation when electromagnetic wave travels in the spherical dielectric stratum formed by the Earth‟s surface and the lower edge of the ionospheric plasma. The dielectric shell has a small thickness of about 60–100 km in comparison with the Earth‟s radius of ~6366 km. This non-conducting layer forms a spherical cavity resonator, with two sets of eigen-values. One of them corresponds to the global electromagnetic resonance predicted by Schumann [1952], which is often regarded as SR. Its peak frequencies (frequencies of maxima in the power spectra of natural ELF radio noise) are approximately 8, 14, 20, Hz, etc. At such low frequencies, only transverse electromagnetic (TEM) wave propagates in the cavity having the non-zero vertical electric Er and horizontal magnetic H field components. If we account for the finite ground conductivity, a small longitudinal electric field E appears. It is clear that we use the spherical polar coordinate system (r, , ) with the origin at the Earth‟s center and the polar  = 0 axis directed to the vertical electric dipole source. Owing to small losses and a great wavelength, the resonance signals can multiply & circle the globe, so that direct and antipodal waves interfere forming a set of maxima in the power spectrum of vertical electric or horizontal magnetic fields positioned at the above-mentioned frequencies. Lightning strokes of the global thunderstorms occurring at the rate of 50–100 discharges per second serve as the source of natural ELF radio noise. SR might be regarded as “longitudinal” one: it occurs in the waves propagating along the Earth‟s surface. The second type of resonance corresponds to vertically propagating radio waves, therefore, it is called the transverse resonance. Relevant waves are captured between the ground and ionosphere, so that their basic frequency depends on the ionosphere height and is equal to 1.7 International Journal of Electronics and Applied Research (IJEAR), Vol.3 Issue 2, December, 2016 Published online (http://eses.co.in/online_journal.html) ISSN 2395 0064 Copy right @ESES IJEAR 2 kHz when the thickness of the ground–ionosphere gap is 88 km. Transverse resonance is a local phenomenon, because it is not linked with the round-the-world propagation. In radio engineering, the eigen-values of transverse resonance are regarded as cutoff frequencies of the modes propagating in the Earth–ionosphere duct. The basic frequency is the cut-off frequency of the first-order mode, the second-order mode has the cutoff equal to the doubled basic frequency of transverse resonance, etc. Obviously, only the basic transverse resonance frequency definitely belongs to the ELF band, while the higher-order modes are found within the very low frequency (VLF) band.