Geoscience Reference
In-Depth Information
6.4
ULF Electromagnetic Noises
6.4.1
Main Sources of the ULF Noises
The global electromagnetic resonances which have been considered in this section,
cover a wide frequency range from several mHz to 30-35 Hz. The first Schumann
resonances cover the overall range from 7-8 to 30-35 Hz, which are in the ELF
frequency band. The IAR eigenfrequencies lie in the range from 0.5-0.25 to 3-5 Hz.
The FLRs and the cavity mode eigenfrequencies are below this range since they
typically cover the interval 10
2
-10
3
Hz. In what follows we focus on the natural
ULF noise, that is at the frequencies which are even smaller than above resonant
frequencies covering the range of the global electromagnetic resonances.
The Earth electromagnetic field is subject to a variety of random forces such as
the variations of solar radiations, incident MHD waves and global magnetospheric
resonances, fluctuation of the ionospheric currents, changes in the world thunder-
storm activity, and so on. A variety of magnetospheric MHD waves acting on the
Earth ionosphere give rise to a wideband spectrum of electromagnetic perturbations,
which can be detected on the ground surface. Throughout the frequency range from
HF to ULF the flux density of natural magnetic variations increase with a decrease
in frequency in such a way that the amplitude of the spectral density varies from
10
21
-10
24
W/(m
2
Hz) at frequency 10
9
-10
10
Hz up to 10
3
-10
1
W/(m
2
Hz) at
frequency
10
3
Hz (Lanzerotti
1978
). Figure
6.11
taken from Lanzerotti et al.
(
1990
) shows spectra of background magnetic variations measured in the wideband
frequency range, which cover the ten-decades from 10
5
to 10
5
Hz. Interestingly
enough the noise in the ELF/VLF range is an overall approximate inverse relation
between the noise amplitude and frequency (Lanzerotti et al.
1990
; Fraser-Smith
1995
). This implies that there is an overall approximate inverse relation between the
noise power amplitude and frequency. Notice that a power law spectrum of noise,
which is referred to as the class of 1=f noise, or flicker noise, is usually observed
in all electric devices over a very broad frequency range (e.g., see Rytov et al.
1978
; Weissman
1988
). There exists other tendency in the frequency range from
10
5
to 10
1
Hz where in the first approximation the noise amplitude is inversely
proportional to f
1:5
. As indicated in Fig.
6.11
, the spectrum of the noise amplitude
in the intermediate interval can be approximated by a power law proportional to f
n
with the exponent n laying in the range 1:0-1:5. However, the value of n appears to
vary considerably, depending on the case study, measurement technique and on the
instruments arranged at the ground-recording station.
Knowledge of these tendencies for the natural low-frequency noise is of special
interest in geophysical studies, since it gives information about spatiotemporal
variations of the natural ULF electromagnetic noise and their source mechanism.
This knowledge is also important from a scientific point of view, because, as pointed
out by Fraser-Smith (
1995
), it is not understood at present why there exists such a
relation between the ULF noise amplitude and frequency.
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