Geoscience Reference
In-Depth Information
behave randomly. Effective charges may rotate around the vertical axis because of
vortices arising beyond the shock front.
The dipole and quadrupole terms in Eq. (
11.32
) were comparable in some
experiments. In the case of explosives with mass of 50 kg the dipole term begins
to be prevalent over the quadrupole one at the distances larger than 57 m. It is not
surprising because the dipole term decreases more slowly away from the epicenter.
So the quadrupole law r
4
is applicable in the near zone of explosions which is
limited by some critical radius r
0
D j
Q
x
z
=d
z
j
l .q
1
h
1
C
q
2
h
2
/=.q
1
h
1
q
2
h
2
/.
In the area r>r
0
the dipole term predominates and the field amplitude decreases as
r
3
. It is usually the case that the vertical separation of electric charges and dipole
moment increases with time at least when the time is greater than 1-10 s (Adushkin
and Soloviev
1996
). This implies that the critical distance r
0
decreases with time.
The nuclear explosions are frequently accompanied by the generation of light-
ning discharges. The five upward-propagating discharges were detected during a
thermonuclear detonation “Mike” test with TNT equivalent 10.4 Mt (Uman et al.
1972
). The detonation was in the large ground-based hall at Eniwetok Atoll in the
Pacific on 31 October 1952. It appears that the lightning discharges were initiated
from instrumentation stations slightly above sea level. The major cause of the
electric field generation is believed to be the flux of Compton electrons produced by
nuclear detonations. For the detonation with such a TNT equivalent the estimated
initial number density of ionized particles reaches the value about 10
15
pair/cm
3
,
which is sufficient for electrical breakdown in the ionized air (Uman et al.
1972
).
Laboratory tests and numerical simulations showed that the breakdown conditions,
branching and configuration of the discharge channels are determined by the spatial
charge distribution in the exposed atmosphere (Hill
1973
; Grover
1981
; Colvin et al.
1987
; Williams et al.
1988
).
For the powerful surface explosions the strong quasistatic electric fields can be
interpreted in terms of a vortex ring of the heated gas and dust. The lifting of the
vortex ring in the atmosphere is caused by Archimedian force which results in the
electric charge separation between the vortex ring and dust column. Holzer (
1972
)
has observed an enhancement of the Earth electric field by as much as 60 V/m for the
time of the vortex lifting in the atmosphere (2-3 min). The numerical calculations
have shown that the electric field at the top of the dust column can reach the
breakdown level in the air as the column height increases up to 1-2 km (Surkov
2000
). It appears that the lightning can be initiated in the dust cloud of explosion
similar to that occurring in the volcanic ash cloud. The IC lightning discharges
can explain the sharp peaks which are occasionally observed at the background of
quasistatic electric field produced by explosions (Soloviev and Surkov
2000
).
Search WWH ::
Custom Search