Geoscience Reference
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Fig. 4.2
Experimental power spectra, which contain the Schumann resonances. Arrival Heights,
Antarctica (AH), Sondrestromfjord, Greenland (SS), Stanford, California, USA (SU). Taken from
To interpret this phenomenon one may assume that the Schumann resonances
arise from the large-scaled electromagnetic waves propagating inside the atmo-
spheric waveguide around the Earth. Typical wavelength of these waves is of the
order of the Earth's radius and thus is much greater than the atmospheric altitude d.
The eigenfrequencies, f
n
, of the Schumann resonances can be roughly estimated
in terms of the fact that whole numbers of electromagnetic wavelength must keep
within a circle 2R
e
, where R
e
is the Earth's radius. Hence we get the estimate
f
n
cn=.2R
e
/
7:5n (in Hz), which is close to that given by Eq. (
4.26
),
especially as for the large numbers n.
The tangential component, E
, of the electromagnetic field caused by a CG
lightning discharge is small inside the resonator since it becomes zero at the
resonator sides. The radial electric component E
r
is perpendicular to the direction of
wave propagation. This component of the quasi-transverse electromagnetic normal
mode dominates in the resonance cavity. As far as d
R
e
, the resonator is partly
similar to a flat waveguide between two infinite parallel conductive plates. In such
a case the perpendicular component E
r
is practically insensitive to the waveguide
altitude.
Worldwide thunderstorm activity is believed to be the main source/mechanism
for excitation of the Schumann's spectra. The Schumann resonances were originally
measured by Balser and Wagner (
1960
) and have been extensively studied by a
number of authors (e.g., see Galejs
1965
,
1972
; Bliokh et al.
1980
; Nickolaenko
and Hayakawa
2002
). In Fig.
4.2
the Schumann's spectrum measured in pT/Hz
1=2
is
plotted. As is seen from Fig.
4.2
, the observed resonance frequencies are somewhat
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