Earth-Ionosphere Cavity Resonator - Ultra and Extremely Low Frequency Electromagnetic Fields - page 130

the difference is due to a modulation related to the local height of the ionosphere
at respective observing sites. It is known that the day-night asymmetry in the
height of the D region is mainly due to diurnal variations in the solar UV and
Roentgen radiation. On the basis of this assumption, Sentman and Fraser ( 1991 )
have shown that the function describing the global lightning may be determined
once the diurnal modulation factor is removed. Further improvement has been
developed by Nickolaenko et al. ( 2010 ) in order to separate the UT (universal time)
and LT (local time) variations in Schumann resonance data.
As would be expected, considering the major role played by the global lightning
in the excitation of Earth-Ionosphere resonance cavity, the Schumann resonances
exhibit a very high degree of spatial and temporal coherence. For one example,
Holtham and McAskill ( 1988 ) and Sweeney ( 1989 ) have examined the cross-spectra
of the lowest Schumann resonances recorded across baselines of 1;100 and 480 km.
The results show the coherence in the lowest modes at a level about 90-98 %,
which means that the lightning over oceans and in the regions outside the world
thunderstorm centers plays a relatively minor role in the excitation of Schumann
resonances as compared to the contributions of these three continental thunderstorm
centers.
A single impulse from a very large discharge, however, can sporadically excite
the Schumann resonances to an amplitude greater than that due to incoherent sum
of random fields caused by the global lightning activity. Such large discharges can
serve as the sources of transients called Q-bursts, which can be simultaneously
detected at very far distances (Ogawa et al. 1967 ; Sentman 1989 ; Nickolaenko et al.
2010 ). It is usually the case that a Q-burst is a damping quasi-periodic oscillation
at one of the eigenfrequencies, more usually at the lowest frequency near 8 Hz.
Figure 4.5 shows typical recordings of the Q-bursts recorded in California in 1985
(Sentman 1989 ). The power spectrum of the Q-bursts is shown to be concentrated
at the Schumann resonance frequencies. As is seen from Fig. 4.5 the amplitude of
the oscillations decreases exponentially for 0:5 s, that is, at a rate corresponding
to the Q-factor of the Earth-Ionosphere cavity.
As we have noted above, the probability of C CG lightning dominates that
of CG lightning when the charge moment magnitude exceeds several hundreds
C km. These huge C CG flashes can play a significant role in sporadically exciting
the Schumann resonances since this kind of flashes stands well above the global
lightning population for short periods of time. Perhaps, the same conclusion can be
applied to the variations of atmospheric electric field. Recently Füllekrug ( 2004 ) has
reported that the intense positive lightning discharges result in a weak decrease in
intensity of the global atmospheric electric field.
Recently a distinctive pattern of Schumann resonances has been measured on
the low earth orbiting C/NOFS satellite within the altitude region of 400-850 km
sampled by the satellite (Simoes et al. 2011 ). The C/NOFS satellite was equipped
with instrumentation for measuring three-component electric field. Typical ampli-
tudes of the first peak was 0:25Vm 1 Hz 1=2 whereas the sensitivity of
the electric field onboard measurements was 10 nVm 1 Hz 1=2 in ELF range.
These amplitudes are about three orders of magnitude lower than that observed