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11
ULF-Sounding of Magnetosphere and Earth
11.1 Introduction
The investigation of the pulsation spatial structure was based on the solution
of direct problems of MHD-wave propagation. In this chapter, we shall show
how the developed concepts can be adapted to the inverse problems, to deter-
mine the magnetospheric cold plasma distribution and to study the Earth's
crust conductivity by using either ground or satellite ULF-observations.
We will consider a method of ground-based magnetospheric plasma moni-
toring based on the FLR-theory (see Chapter 6). In this method the distribu-
tion of Alfven velocity and, thus, plasma density, along a field line is obtained
from the FLR-frequencies. To this end, the inverse problem is reduced to a
spectral problem. The FLR-frequencies are found from the geomagnetic pul-
sations.
The Earth's crust conductivity is determined by methods known as mag-
netotelluric sounding (MTS)([10], [32]). The method is based on simultaneous
measuring of electric and magnetic fields in the ULF-range. Numerous papers
and monographs have been devoted to this topic. However, little attention has
been devoted to the influence of magnetospheric peculiarities of MHD-wave
propagation and excitation. We shall study the role of resonance magnetic
shells in interpreting data of ground-based MTS as well as the possibility of
an above ionosphere MTS from low-orbit satellites.
11.2 Inverse Problem of FLR
Obayshi [26] and Dungey [14] were the first to suggest that the plasma para-
meters can be reconstructed from the FLR-spectrum of a field-line. While in
general this is a very dicult task, under ideal MHD, the spectrum of the mag-
netosphere oscillations splits into two parts (see Chapter 6): a discrete spec-
trum of global magnetospheric oscillations and a continuous FLR-spectrum.
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