Geoscience Reference
In-Depth Information
The atmospheric layer between the highly conducting terrestrial surface
boundary and the conducting but dissipative ionosphere forms a dissipative
spherically concentric cavity, the Earth-Ionosphere cavity, that can serve as a
resonator for electromagnetic waves with wavelengths comparable to the Earth
radius. As shown below, the resonant spectra of these waves fall in the ELF range.
The main mechanism for excitation of the resonance spectrum is the
electromagnetic energy stemming from the global thunderstorm activity (e.g.,
see Sentman 1995 ; Nickolaenko and Hayakawa 2002 ; Hayakawa et al. 2011 ). The
lightning discharges radiate electromagnetic impulses with broadband spectrum.
The lowest portion of such a spectrum is responsible for excitation of the quasi-
steady electromagnetic waves in the Earth-Ionosphere resonance cavity. The
total resonant spectrum is an incoherent superposition of contributions from the
individual lightning discharges occurring over the globe. This phenomenon, called
Schumann resonances, constitutes a prevailing part of the natural background
electromagnetic spectrum over the frequency range 5-50 Hz.
It was Schumann who first studied and predicted the resonance properties of
the Earth-Ionosphere cavity. The early study of the Schumann resonances was
stimulated in part by the U.S. Navy's interest in investigating the ELF band for
possible use in submarine communications (Wait 1974 ; Sentman 1995 ). A large
amount of experimental study on this problem was performed between 1965 and
1982 when Schumann resonances research was at its peak (e.g., see review by
Bliokh et al. ( 1980 ) and references therein). The overview of these studies and
recent results, including the theory of relevance to Schumann resonances, have
been summarized in the topic by Nickolaenko and Hayakawa ( 2002 ). The reader is
referred to that topic and to the texts by Budden ( 1962 ), Galejs ( 1972 ), Wait ( 1972 ),
and Bliokh et al. ( 1980 ) for details about more complete theory of the Schumann
resonances.
A number of different theoretical models have been used to describe the reso-
nance spectrum and sources of the cavity excitation. Following Schumann ( 1952a , b ,
1957 ) we consider a simplified model of the resonance cavity. In this model the
atmosphere is considered as an insulator while the Earth and the ionosphere are
supposed to be perfect conductors, so that the Earth-Ionosphere forms a spherical
capacitor, which contains the resonance cavity bounded from below and from above
by the perfectly conductive walls.
CG lightning discharges are believed to be the principal source for excitation of
the Schumann resonances (Sentman 1995 ). Since the vertical CG return stroke is the
most effective emitter of the electromagnetic waves, consider first a single vertical
current impulse as a source for the cavity excitation. All the values are supposed
to vary as exp . i!t/, where ! is the frequency. The Maxwell equations ( 1.1 )
and ( 1.2 ) for the atmosphere can thus be rewritten as
i!
c 2 E ;
r B D 0 j s
(4.1)
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