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In-Depth Information
14
Active Cloud Releases and MHD-Emission
14.1 Introduction
In this chapter we describe MHD-perturbations generated through the release
into the ionosphere of plasma producing products. Careful consideration needs
to be given to the dynamics of the release cloud and to various geophysical
effects associated with this. Detailed discussions of these problems may be
found in ([1], [2], [3], [4], [5], [6], [10], [11], [13]). Our purpose in this chapter
is to show, based on a simplified dynamic model, how electromagnetic pulses
can be generated by a man-made ionospheric plasma release.
Let us consider an injection of plasma producing compounds at heights
of 150-200 km. Initially the emission speed of the gas cloud is about 2-3
km/s. Due to the high cloud density, conditions typical for an ionospheric
dynamo area hold and therefore intensive horizontal dynamo currents can be
generated.
First, the emitted release products (initially, the cloud is composed of
neutral particles with speed
v
n
) induce an electrical current
j
=
σ
P
v
n
B
0
+
σ
H
B
0
B
0
×
v
n
c
×
B
0
.
c
×
Since the release plasma is more conductive than the ionospheric plasma,
the radial component of the current cannot be compensated and its radial
component is closed by the longitudinal currents
j
out-flowing into the low
ionosphere and magnetosphere.
A conductivity gradient caused by a plasma cloud placed into the ionosph-
eric electric field and current, is the second mechanism that produces longitu-
dinal current. The redistribution of the current systems creates
j
and a po-
larized field
E
.
As such currents are non-stationary, they produce MHD-waves
and in particular Alfven waves. The Alfven wave can traverse large-distances
along the field-lines with no significant distortion or loss. The current pulse
propagates with a velocity close to the Alfven velocity along a force tube that
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