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of electric field originated from the radon input is hardly estimated due to the
deficiency of information about the distribution of radon excess over the seismo
active area.
In conclusion we note that all the effects treated here seem to be sporadic and
case sensitive, since they depend on meteorological conditions, diurnal and seasonal
variation of the air humidity, proximity of the coastline, etc.
10.2
Other Large-Scale Disasters
10.2.1
Electromagnetic Phenomena Associated
with Volcano Eruption
Among a variety of electromagnetic phenomena associated with volcano eruption
the most significant effect is the strong electric field generated by volcanic ash
clouds. Short lightning discharges have frequently been observed inside volcanic ash
clouds (Arabadji
1951
; Anderson et al.
1965
; MacDonald
1972
; Brook and Moore
1974
; Uman
1987
). The majority of intracloud (IC) lightning propagates horizon-
tally. The mean dipole moment of IC lightning is estimated as 100 C
m (Rulenko
1985
), which is approximately 10
3
lower than that of CG lightning discharges. The
energy of IC discharges in volcano clouds is estimated as 10
6
J (Anderson et al.
1965
), that is 10
3
smaller than that of typical CG lightning. The typical lightning
length is about 8-10 m while the CG lightning channel length varies within several
km. This implies that the spatial separation of electric charges in the volcanic ash
cloud is much smaller than that in conventional thunderclouds. The charge density
in the volcanic cloud is assumed to be of the order of 10
11
-10
12
C/cm
3
.The
experimental evidence of the volcanic lightning is displayed in Fig.
10.10
, in which
the image was obtained by NASA during Sakurajima volcano in southern Japan on
March 11, 2013. As is seen from this figure, the lightning occurs between the hot
magma bubbles and the volcanic ash clouds above the volcano's tip.
The generation of the electric charges occurring in the volcanic ash cloud is
believed due to the following mechanisms: (1) Electrification of the smallest lava
particles (with sizes up to 10
7
cm) during explosive fragmentation of the lava at
the initial stage of explosion. Above a volcano crater the number density of such
particles reach a value 10
9
m
3
(Zemtsov et al.
1976
); (2) A friction between
ash particles and walls of volcano vent during the upward motion of ash-gas flux;
(3) Electrification of ash particles due to their collisions and friction as well as due
to the friction between the particles and air fluxes (Lenchenko
1988
).
Interestingly enough, the strong electric field can take place near the point where
the lave comes in contact with sea water. Brook and Moore (
1974
) have measured
the electric field amplitude about 7 kV/m at the distance 100 far from this point.
Weak electric discharges and jumps of electric field following 0.6-1.4 s have been
observed during a sudden release/outburst of vapor-drop mixture from the crater
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