Geoscience Reference
In-Depth Information
Chapter 6
Magnetospheric MHD Resonances
and ULF Pulsations
Abstract
This chapter deals with the low-frequency MHD oscillation of the whole
magnetosphere and ULF pulsations including their origins and magnetospheric
plasma instabilities. We discuss briefly magnetospheric models and the generation
of field-line resonances (FLRs) and cavity modes. Properties of MHD waves
propagating in the solar wind are covered. In the remainder of this chapter we
examine source mechanisms of natural electromagnetic ULF noises.
Keywords
Cavity mode Field-line resonances (FLRs) Plasma instabilities
Space weather ULF electromagnetic noises
6.1
Structure of Global Magnetospheric Oscillations
6.1.1
An Axisymmetric Magnetosphere Model
It is customary to believe that the MHD oscillation of the whole magnetosphere was
originally studied by Dungey (
1954
) who derived equations for the eigen oscillation
of an axisymmetric magnetosphere. The normal magnetospheric MHD oscillation,
which is independent of azimuthal angle ', can be split into two practically
uncoupled modes: toroidal and poloidal modes depending on their polarization.
In the poloidal modes, the electric field oscillates in the azimuthal direction, while
the magnetic field and plasma velocity pulsate across magnetic shells. By contrast,
in the case of the toroidal modes, the electric field is in the meridional plane,
while the magnetic field and plasma velocity oscillate in the azimuthal direction.
These types of the magnetospheric MHD oscillation have been studied intensively
for several decades and much is now known of their properties (Radoski
1967a
,
b
;
Radoski and Carovillano
1969
; Cummings et al.
1969
; Krylov and Lifshitz
1984
).
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