Geoscience Reference
In-Depth Information
In order to estimate the intensity of the magnetic pulse, we need to define
q
from (15.23). Let
ζ
=0
.
1,
l
=1,
ω
∗
=2
10
−
2
s
−
1
, z
1
= 6 (see Fig. 15.1),
corresponding to
z
= 120 km (height of the maximal ionospheric conductivity
for the dayside ionosphere). Let also
c
s
=3
.
3
×
10
4
cm/s, (
ρ/ρ
0
)
1
/
2
×
≈
10
4
,
then
10
−
6
V
0
t
0
.
q
≈
2
×
Let us illustrate this through a simple example. Here, ground motions
during strong earthquakes produce an acoustic pulse. The vertical ground
displacement during a strong quake is approximately 1 cm for a typical time
of 1 s. Then the intensity of the magnetic variations caused by such vertical
motion is
b
m
=10
−
6
G=0
.
1nT
.
The intensity of the induced field is pro-
portional to the square of the signal duration. This means that long-period
oscillations can produce visible magnetic variations with intensity comparable
with background magnetospheric and ionospheric variations.
For our estimations, we assume that the incident wave is a plane wave. The
considered model and therefore all conclusions found here are, strictly speak-
ing, only valid for the equatorial regions and for spatially extended sources.
The ionospheric parameters used in the calculations are typical for the dayside
ionosphere in the period of maximal solar activity.
Localized Impulsive Source
The paragraphs above have given some indications of the small influence, at
least at ionosphere levels, of charged particles and Ampere's force on the wave
propagation over neutral particle gas. Therefore, as is usually done in such
problems, the neutral component perturbations are calculated first. Then,
the induced electrical dynamo-field, current and, finally, the magnetic field
are estimated.
Let us consider only qualitatively a situation when the local source is an
explosion carried out in the atmosphere. The linear scale
l
0
of the explosion
of the energy
Q
0
is
l
0
=(
Q
0
/p
0
)
1
/
3
,
where
p
0
is the undisturbed atmospheric
pressure at the explosion height. The process goes to the linear stage when
the radius of the perturbed region is more than
3
l
0
.
Therefore, we
assume that the effective source of disturbances in the atmosphere is a sphere
of radius
R
0
. For example, if the energy
Q
0
of a ground blast is
Q
0
= 100
tTNT=4
.
2
.
R
0
≈
10
18
erg then
l
0
≈
10
4
cm and
R
0
≈
10
4
cm for
p
0
=
×
2
×
6
×
cm
−
2
.
The pulse produced by the blast of radius
R
0
and energy
Q
0
propagates
in an inhomogeneous atmosphere. The problem of wave propagation in a neu-
tral atmosphere from a local impulsive source reduces to consideration of
two wave modes - acoustic and acoustic-gravity waves. The acoustic grav-
ity branch exists at frequencies of less than the Brunt - Vaisala frequency
ω
B
=[(
γ
10
5
dyn
N
m
kT
0
=4
.
2
×
·
1)
g/γH
]
1
/
2
.
Here
γ
is the polytropic exponent. For
H
=8km,
−
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