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This result differs from that obtained by Kopytenko and Nikitina (
2004a
,
b
)by
the factor
r
=
m
because they left out of account the conductivity of the rock
surrounding the vent and hence they overestimated the magnitude of GMPs.
Applying Eq. (
10.53
) to the case of magma upwelling, one may estimate the
amplitude of magnetic perturbations in the vicinity of the volcano vent as follows:
ıB
max
0
r
a
2
B
0
V
max
=r.
The back conduction current is in turn driven by the transverse electric field
resulted from the charges at the vent walls as shown in Fig.
10.11
a. A schematic
plot of the total current density lines is shown in Fig.
10.11
b. It should be noted that
inside the vent .r < a/ the total current
j
is directed in opposition to the
E
, that is
r
j
H
m
C
r
;
E
D
j
H
m
C
r
:
j
D
(10.54)
In the framework of the model both fields,
j
and
E
, are constant inside the vent. In
the surrounding rock they have a power-law decrease with distance .r>a/
E
r
D
j
j
H
j
a
2
sin
.
m
C
r
/r
2
; and E
D
E
r
cot :
(10.55)
In order to estimate the magnitude of GMPs in the vicinity of a volcano we
choose the typical parameters
r
D
10
3
-10
2
S/m, B
0
D
5
10
5
T and r
D
5 km.
Taking the following values of the magma parameters V
max
D
5 m/s and a
D
0.1-
1 km (Kopytenko and Nikitina
2004a
,
b
), we obtain the estimate of maximal effect
related to the active period of volcano activity: ıB
max
D
0:6
10
3
-0:6 nT and
E
max
B
0
a
2
V
max
=r
2
D
0.1-10 V/m.
The ULF electromagnetic noise caused by the crack generation due to volcano
activity can be as much as that due to an EQ since the spatial scale of the
fractured zones appears to be comparable. The above estimates derived for the
microfracturing and GMPs can be applied to this case. However, there may
be expected the enhancement of the effect in the vicinity of volcano crater because
in this region the underground cracked zones can be closer to the ground surface.
In order to estimate this effect, one can substitute the shorter distances in the above
equations.
The effect of the volcano on the ionosphere is believed to be due to IGWs and
acoustic waves generated during the volcano eruption. Notice that the IGW can
be excited not only by the volcanic explosion but also because of air temperature
and density gradients in the vicinity of volcano crater. These waves can disturb
the ionospheric plasma density and generate electric currents thereby exciting low-
frequency electromagnetic perturbations. The variations of total electron content
(TEC) in the ionosphere with period from 16 to 30 min have been observed during
the Mount Pinatubo volcano eruption on June 15, 1991 in Taiwan (Cheng and Huang
1992
) and around Japan (Igarashi et al.
1994
). Heki (
2006
) has recorded the TEC
variations 12 min after the Asama volcano eruption, central Japan in September
2004. The period of the ionospheric variations was about 1.3 min, which is close
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