Geoscience Reference
In-Depth Information
It follows from different models of the Earth-Ionosphere resonator that the set of
functions g ' contains singularities, which correspond to the Schumann resonances
(e.g., see Sentman 1995 ). Given the functions g ' and the lightning distributions over
the globe, the set of Eqs. ( 4.47 )-( 4.49 ) describes the spectral correlation matrix and
power spectra of the electromagnetic variations due to global thunderstorm activity.
One may take into account only the most active world thunderstorm centers situated
at tropics in order to give a rough estimate of this power spectrum.
One further comment should be made that the IC (intra-cloud) lightning dis-
charges cannot, in general, be observed at distances as great as CG lightning
(see Chap. 3 ). Although the IC lightning discharges of both positive and negative
polarities are strongly prevalent over CG ones, it appears that the IC lightning
contribution in the Schumann resonance frequency range is smaller as compared
to CG because the CG lightning is a much stronger radiator at these frequencies due
to the presence of continuing current.
There should be emphasized three important properties followed from Eqs. ( 4.47 )
and ( 4.48 ). (1) The total resonant power spectrum is an incoherent superposition of
contributions from individual lightning discharges occurring over the globe. (2) The
spectral matrices produced by individual thunderstorms are summed independently
to provide the total spectral matrix of random process. In a similar fashion we may
add variances produced by independent random variables. (3) The spectral matrix
produced by an individual thunderstorm is directly proportional to D M 2 E .
In conclusion we note that the functions g ' ;! given by Eq. ( 4.37 )are
proportional to the function F .!/ describing the profile of individual or multiple
return strokes. This implies that the spectral densities of Eqs. ( 4.47 ) and ( 4.48 )
are directly proportional to j F .!/ j
2 ; that is, to the power spectrum generated by
a single lightning discharge. In a more accurate model, one should distinguish
between F n .!/ and F p .!/ as well as between M n and M p when describing the
negative and positive lightning discharges, respectively.
4.3.3
A Role of Positive and Negative Cloud-to-Ground (CG)
Lightning
As has already been stated, most CG lightning discharges are negative, that is, the
lightning current transfer negative charges from the thundercloud to the ground. The
CG current is thus upward directed. Only about 5-10 % of global CG lightning
activity results from C CG lightning, which transfer the positive charge to ground.
Despite their infrequent occurrence compared to the negative flashes, the global
C CG lightning can be of primary importance in generating the ULF/ELF power
spectrum and Schumann resonances since their charge moment and continuing
current are, on average, larger than those of the CG lightning.
The positive ground flashes usually stand well above the global lightning
population for their large current/charge moment. With the charge moment change
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