ULF-Sounding of Magnetosphere and Earth - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 331

400

200

0

0

− 40

− 80

0 4 8 12 16 20

T 1/2 [s 1/2 ]

Fig. 11.7. Plots of the impedance and apparent resistivity near resonance shell
as functions of T 1 / 2 . Solid curve refers to the apparent resistivity ρ ap ( T ) for the
normally incident initial wave. The long dashed and short dashed lines are the
apparent resistivities ρ ap ( T ) calculated for latitudes Φ =60 ◦ (long dashed) and
Φ =55 ◦ (short dashed) where the resonance periods are 100 s and 46 s, respectively

11.4 The Satellite Electromagnetic Sounding of Earth

Sounding on a Spatial Harmonic

There are two main di culties inherent in the method of ground magne-
totelluric sounding. The first is the existence of horizontal subsurface inho-
mogeneities. Such inhomogeneities cannot only complicate, but often make it
impossible to interpret the observed data. The diffraction field of the waves
scattered on the inhomogeneities, and the reflected wave from the ground lay-
ers superimpose on the initial wave field. If there are inhomogeneities, the
approach employed in the different geophysical methods is moving away from
the irregularities and using height observations. The other complicating factor
is the necessity of prior assumption about the spatial structure of the initial
wave. For example, an assumption of a plane wave which is incident nor-
mally on the ground. Moving the observations above the ionosphere, solves
the second problem due to the peculiar ionospheric reflection of an Alfven
wave.
The method of satellite electromagnetic sounding of the Earth may be
based on the investigation of a horizontal homogeneous model of the media