MHD After-Effects of a Sound Impact - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 391

the isothermal atmosphere, the energy density ( ρ 0 + δρ ) v 2 must be con-
served. Here ρ 0 is the atmospheric density at height z , δρ is the den-
sity perturbation in the acoustic wave and v is the gas velocity in the
wave. With an accuracy up to terms of second order, ρ 0 ( z ) v ( z ) 2 =const

or

ρ ground
ρ ionosphere

1 / 2

v ionosphere
v ground ∼

.

Let us suppose that a 100 s weak ground wave with 1 mm / s initial gas ve-
locity reached the 100 km altitude. Then the ratio of the gas velocity in the
wave to the initial wave is δv ionosphere /δv ground ∼ 10 4 and the displacement of

the ionospheric gas is 1 km. Due to such unique feature, even rather small

large-scale ground perturbations can become visible at the ionospheric heights.
Moreover, the wave amplitudes may be so strong that nonlinear effects will
become remarkable.
The entrance of the acoustic wave into the ionosphere causes a change
of the ionospheric parameters, disturbs chemical equilibrium between the
ion-neutral ionospheric species, and results in a change of the ionospheric
turbulence scale. An important point is that eventually it can change the
Pedersen conductivity and as a sequence of this, a change of the reflected
ionospheric properties and Q -factor of the magnetospheric Alfven reso-
nator.
The motion of the neutral gas in the acoustic wave results in an appear-
ance of the ionospheric electric fields and currents producing an Alfven pulse
going up into the magnetosphere to the conjugate ionosphere. Besides the dy-
namo field, appearing small-scale irregularities increase the Pedersen effective
conductivity (see Chapter 10). Permanently existing large-scale ionospheric
current systems are redistributed in the vicinity of the region with anomaly
Pedersen conductivity that in turn results in the longitudinal currents of the
Alfven wave.
Thus, as a third possibility, in addition to the sources discussed in previous
sections, there may be mechanical motions of the ionosphere convertible to the
MHD-waves. We will consider features of the acoustic-gravity waves and ULF-
magnetic variations caused by an oscillating and impulsive on-ground and
under-ground sources like earthquakes, large-scale atmospheric phenomena,
air and underground explosions, etc.
First, an elementary model of a plain low-frequency pulse is considered
propagating vertically upwards over a non-uniform atmosphere. This pulse
in turn excites an electrical current and magnetic pulse. Estimations of a
magnetic pulse initiated by a local source are also given. The consideration of
the 1D case of the wave propagating upward is especially useful in estimates of
the maximal magnetic signal which can be get from various acoustic sources.
The results of an experiment that dealt with acoustic action on the ionosphere
and magnetosphere are shown in the last section.

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