Geoscience Reference
In-Depth Information
15
MHD After-Effects of a Sound Impact
Ionospheric HF-heating and releases of chemically active products discussed
in the preceding chapters are only of interest to us because of the option to
use them to study the magnetosphere-ionosphere plasma and some features
of the MHD-wave generation and propagation. As a matter of fact, these
mechanisms are purely anthropogenic. It is dicult to imagine situations
in extra-terrestrial space in which naturally occurring strong natural radio-
emissions that provide noticeable radio-heating, or natural plasma injections
much denser than the plasma densities exist.
The third mechanism of MHD-wave generation, which we will discuss next,
is an important part of the lithosphere-magnetosphere coupling. Here, we are
dealing with an atmospheric wave due to large-scale high-energy releases, ei-
ther in the low atmosphere or on Earth. Some of the processes contributing
to the dynamics of the atmosphere, ionosphere and magnetosphere are caused
by weather processes, earthquakes, volcanic eruptions, winds in mountain re-
gions, etc. All these processes create acoustic and acoustic-gravity waves that
can reach ionospheric heights. For a deeper understanding of neutral wave
propagation in the atmosphere, however, we would refer the reader to [21]
where a review and an analysis of the current status of both theoretical and
experimental studies of long-range sound propagation in the atmosphere are
given. Here we restrict our consideration to a basic sketch of a neutral wave
propagation, describing the key facts of the process.
Bombarding the ionosphere with acoustic waves causes macroscopic mo-
tion of ionospheric layers and perturbation of the chemical kinetics of ionized
and neutral ionospheric species. The acoustic wave reaching the ionosphere
also gives rise to temperature and pressure perturbations. In principle, it can
perturb the chemical equilibrium between ion-neutral ionospheric species. And
finally, the wave entrance can be accompanied by displacements of ionospheric
layers.
Let us estimate velocities and displacements within the ionosphere at the
acoustic wave beam axis without considering a dissipation. In the case of
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