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frequency displacement of the FLR-oscillations from plasma compression and
from longitudinal currents will be of the same order at any distances from the
resonance shell.
12.2 TEC Modulation by an MHD-Wave
At the above ionosphere radio wave sounding, the radio wave transmitted
from the magnetosphere penetrates through the ionosphere and reaches a
ground receiver. The geomagnetic field results in the ionosphere becoming an
anisotropic medium. It means that there are in general two refractive indices
associated with two different wave polarizations. Suppose that a linear radio
wave is transmitted from the magnetosphere. The linear wave may be resolved
into two circular components with an opposite rotation sense. Since the re-
fraction indices (
n
) of these waves are different then their phase velocities
are different as well. Thus polarization plane of the downgoing wave turns
below the ionosphere with respect to the initial polarization plane. The value
of the angle (the Faraday rotation angle) is defined by the total electron con-
tent within the column between the magnetospheric emitter and the ground
receiver.
Short-period fluctuations of
N
T
were discovered in experiments to deter-
mine the Total Electron Content
N
T
(TEC) of the ionosphere with the help
of a radio beacon on a geostationary satellite ([7], [11], [4]). The appearance
of these fluctuations was associated with the intensification of
Pc
4 pulsations.
The so-called differential Doppler method with very high sensitivity was used
to determine
N
T
along the radio path. Comparison of simultaneous TEC
records and of the
H
(or
D
) pulsation-field component demonstrates a clear
connection [11]. The mechanism of plasma compression in the wave field stim-
ulated by the presence of a longitudinal MHD magnetic component
b
proved
the most effective [13]. The differential Doppler method enables integral elec-
tron concentration along a radio beam between emitter (
S
) and receiver (
R
)
to be determined from observations of satellite radio beacon signals:
S
N
T
=
N
e
dz .
(12.42)
R
Determination of
N
T
from Faraday Rotation
The method based on measuring the Faraday rotation angle provides infor-
mation on the weighted-mean value of TEC. The rotation angle of the polar-
ization plane of the VHF radio wave on the way between the geostationary
satellite and the ground depends on the number of free electrons along the
ray path. A linearly polarized wave in an anisotropic plasma can be pre-
sented as sum of the right-hand and left-hand polarized waves with different
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