Electrodynamic Properties of Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 40

90

80

70

60

50

40

30

20

10

0

1E-4 1E-3 1E-2 0.1 1 1E+1 1E+2 1E+3 1E+4

conductivity [s −1 ]

Fig. 2.9. The distribution of the atmospheric conductivity as a function of altitude

2.4 Summary

In general, in problems of interaction of MHD-waves with the Earth-
atmosphere-ionosphere-magnetosphere system, this system may be replaced
by a layered system, each layer with its own specific properties (see Fig. 2.10).
The ground conductivity is so high compared with the ionospheric conduc-
tivity, that it can be considered to be a metal conductor. Its conductivity
depends both on the depth and on the horizontal coordinates. It is known,
for example that there exist two kinds of crust conductivity. The continental
crust has a resistivity l 0 3 Ω

· ≈

m at the depth 100 km and underlined by the

high conductive layer of a resistivity 10 2 Ω

· ≈ ≈

300 km. There
is, on the other hand, substantial evidence for a sharp decrease in electrical
resistivity up to 1

mfrom 100 to

− 10 Ω · m on a global scale at a depth between 400 and 800

km (e.g. [23], [29], [30]).
The atmospheric conductivity differs by 10 orders of magnitude from the
ground conductivity. As a rule, the atmosphere can be considered to be an
insulator. The one exception to this rule is the problem of excitation and
propagation of the atmospheric wave of type TM 0 (see Chapter 8).
Above 60 km, in the ionospheric D -layer, the air density is so rarefied
that electrons can gyrate freely in the geomagnetic field and ions collide so
frequently that their motion is defined entirely by neutrals. Thus, the electri-
cal conductivity of the D -layer is determined by electrons. The D -layer is a
specific region with strong Hall and evanescent Pedersen conductivities.
The E -layer extends approximately from 90 to 140 km. The neutral con-
centration decreases here so that ions can move by the action of the electric