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1
2
O
d
d
a
b
Fig. 11.6
Effective underground (
a
) vertical and (
b
) horizontal dipoles
d
produced by electric
current systems in the homogeneous conducting ground.
1
—current field lines,
2
—radial currents
on the ground surface
for the time of the order of
r
C
f
while the negative half-wave is caused by the
subsequent attenuation of the SW amplitude. The predicted amplitudes of the EMP
at the distances 5-10 km are on the order of several V/m and 1-10 pT, which is
also compatible with observations though the duration of the observed signals is
greater than calculated values.
It appears that the EMP is a combined effect resulted from different sources
such as the high-temperature plasma ball, SW polarization, and perhaps gamma
and neutron radiations. There are a few difficulties in distinguishing these sources
because the direction of the shock-induced dipole is unknown prior to the test; that
is, the dipole direction can be accidental in character.
11.2.2
Residual Electromagnetic Field on the Ground Surface
In what follows we examine the phenomena arising after the abatement of the EMP.
The residual quasi-static magnetic perturbations at the epicenter of surface and
buried detonations have been observed by Stacey (
1964
) and by Undzenkov and
Shapiro (
1967
). One of the first measurements of residual magnetic field has been
made in the region Medeo (USSR) at the distance of 700 m from the detonation
point (Barsukov and Skovorodkin
1969
). The detonation was made in granite,
whose natural magnetization was J
D
5-100 mA/m. The ground-based observation
showed that 1 h 50 min after the moment of detonation the local geomagnetic field
changes by 8 nT and this perturbation is halved 5 h after it. The field relaxation
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