Geoscience Reference
In-Depth Information
8
Propagation of MHD-Beams
8.1 Introduction
The previous chapter discussed field transformation to the Fourier domain
due to the homogeneity in the horizontal plane and time. The reflection and
transformation matrices were found for the wave periodical in space. The thin
ionosphere approximation enabled us to proceed with a minimum computa-
tions to a rough estimate of the general features of the MHD-waves. The
next step, then, is to compute spatial field distributions produced by specific
MHD-wave beams.
When a wave front propagates through the ionospheric plasma, it suffers
reflection back to the magnetosphere and an attenuation due to the Joule
heating. At some distances away from the pierce point of the wave beam, two
waves are mixed on the ground and above the ionosphere. The first one is the
transmitted wave and the second one is the wave reflected from the ground
and the layers within the ground. The phases and intensities of these partial
waves are mixed in different rates and various distances from the beam axis.
The wave can reach the ground observer with various paths. The elec-
tromagnetic field can be seen directly from the place where the beam enters
the ionosphere. This place can be considered as an ionospheric equivalent
source. In the simplest case of the ground modeled by a conductive half-space,
the secondary mirror source arises within the ground. In the layered ground,
there are infinite chains of the imaginary sources. An eciency of radiation
of the 'ground' (imaginary) sources is defined by the ground conductivity and
a distance where the field produced by these underground sources can be
seen. This distance is about the skin depth which, in turn, is proportional to
the root of the wave period. For instance, the skin depth can reach 1000 km
for the oscillations of the 1
10 min period. The phases and intensities of
the 'initial' and 'ground' sources are stacked and form a rather complicated
interference pattern of the waves on small distances from the beam axis.
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