MHD-Waves in Layered Media - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 227

−3

10 0

(a) (b)

b x /b (i)

E y /b (i)

b x /b (i)

− 4

E y /b (i)

10 −1

b y /b (i)

E x /b (i)

b y /b (i)

−5

E x /b (i)

10 −2

0.1 1 10 0.1 1 10

Σ P (Ohm) −1 Σ P (Ohm) −1

Fig. 7.11. Dependence of the electric field on the ground on Σ P . Designations are
the same as in Fig. 7.9. Dependence of magnetic components of the Alfven and
magnetosonic waves on Σ P on the ground. See also the caption to Fig. 7.9

Dependency of horizontal electric fields on the ground as a function of Σ P
is shown in Fig. 7.11a. It is the same as the dependence of b a . Electric fields
in the Alfven wave on the ground and in the ionosphere are turned over π/ 2
contrary to the FMS-wave.
By comparing values of the induced electric components in the ionosphere
and in the ground and corresponding electric currents it is easy to understand
why the ionosphere does not influence the magnitude of the atmospheric and
ground magnetic components of the FMS (dashed lines in Fig. 7.10b and Fig.
7.11b). The skin depth in the conductive ground is

0 . 503

T (s)
σ g Ohm − 1 m − 1 .

d g (km) ≈

For σ g =0 . 1Ohm − 1 m − 1

and T = 100 s , we h av e d g ≈ 16 km. A total

/ √ 2 , and in the ionosphere

J ( g ) E ( g ) current inside the ground | | = d g σ g | |

. The electric field in the ground E ( g ) (Fig. 7.11a) in-
duced by the FMS-waves with amplitude of incident wave b ( i )

J ( I ) E ( I ) | | = Σ P | |

=1nT

E ( g ) 10 − 4 mV / m. The ionospheric electric

and T = 100 s is | |≈ 4 . 76 ×

10 − 3 mV / mfor
Σ P =0 . 1and Σ P = 10 Ohm − 1 m − 1 , respectively. Hence the current in the
ground and in the ionosphere are J ( g )

E ( I ) 10 − 3

fields (Fig. 7.10a) are | |≈ 5 . 55 × and 5 . 97 ×

0 . 54 mA / m, J ( I ) 10 − 4 and ≈ ≈ 5 . 55 ×

10 − 3 mA / m, respectively. The ionospheric current is essentially less
than the ground current. Therefore, the change from day to night condi-
tions does not influence the magnitude of the magnetic component in the
atmosphere (Fig. 7.10b) and on the ground surface (Fig. 7.11b). The homoge-
neous ionosphere, is transparent for the FMS-waves and ionospheric current

5 . 97 ×