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10 0
b x /b (i)
(b
(a)
E x /b (i)
1
b x /b (i)
E y /b (i)
10 1
E y /b (i)
b y /b (i)
−2
b y /b (i)
E x /b (i)
−3
10 2
0.1
1
10
0.1
1
Σ P (Ohm)
1
Σ P (Ohm) 1
Fig. 7.10. (a) Dependencies of the E x and E y components of the electric field in the
ionosphere on its integral Pedersen conductivity. (b) b x and b y components under
the ionosphere as functions of the integral Pedersen conductivity Σ P . Solid lines
correspond to incident Alfven waves. Dashed lines correspond to FMS-waves. For a
description of the medium model, see the caption to Fig. 7.9
electric field caused by such waves should appear on the nightside of the
ionosphere.
Figure 7.10b shows the dependence of the horizontal magnetic compo-
nent in the atmosphere, under the ionosphere, on Σ P . It can be seen that
ionospheric conductivity does not influence the magnitude of the magnetic
component of the FMS-wave (dashed lines). When the amplitude of the ini-
tial magnetic component b i = 1, the magnetic field b a under the ionosphere is
a little bit more than 1. The atmospheric magnetic component produced by
the Alfven wave b x ( b y ) is generally caused by the Hall current j y ( j x ), which
is generated due to the E x ( E y ) component of the incident Alfven wave. The
total magnetic field of the Alfven wave in the ionosphere depends strongly on
the ionospheric conductivity. As it can be seen from Fig. 7.10b, it changes in
roughly 3 orders for change Σ P in 2 orders. It is in the consent with the esti-
mation (7.150) of the total electric field of the Alfven wave. Transition from
the well conductive dayside ionosphere to the nighttime one which has very
small conductivity leads to decreasing of b x and b y components by a factor of
0 . 14 (see (7.151)).
The very low conductivity of the atmosphere means that the currents are
very small. Then (
0 for the magnetic fields under
the ionosphere, produced as the Alfven waves incidence, as and the FMS-
waves incidence. Hence, k x b y
×
b ) z
=4 π/c j z
k y b x
0and b x /b y
k x /k y . In the exam-
ple shown in Fig. 7.10, the ratio b x /b y
10 , i.e., orientation of the hori-
zontal magnetic field under ionosphere does not depend on the ionospheric
conductivity. However, this result is valid only for the horizontally homoge-
neous ionosphere. Ionospheric irregularities can result in change of the wave-
polarization b x /b y .
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