Geoscience Reference
In-Depth Information
k y > 0
k y < 0
ʴ
B
N
4
1
5
L
ʾ
S
2
3
Fig. 6.4 A schematic plot of amplitude variations of FMS wave excited at the magnetopause
and sense of the wave polarization as a function of L-shell in the magnetosphere. 1 —solar wind,
2 —magnetopause, 3 —boundary surface wave caused by Kelvin-Helmholz instability, 4 —plot of
wave amplitude, 5 —resonance field line
Attenuation of the resonance oscillation is basically due to the Joule dissipation
caused by the Pedersen conductivity in the ionosphere. In addition, azimuthal
propagation of the waves leads to the energy losses in the magnetotail.
The Hall conductivity scarcely affects the FLR but it may play a crucial role
in occurrence of the magnetic perturbations under the ionosphere, that is in the
atmosphere and on the ground surface. As the Hall conductivity is ignored, the
incident shear Alfvén wave cannot excite the field perturbation in the atmosphere.
If only the Hall conductivity is finite, the shear Alfvén wave can be transformed
in the ionosphere into both the reflected and transmitted field of the FMS wave.
In other words, the FLR-related field observed on the ground builds up as a result
of the mode coupling in the ionosphere via the Hall conductivity followed by the
penetration of the FMS mode through the atmosphere towards the ground.
A variety of mechanisms of coupling between the shear Alfvén and FMS waves
in a realistic magnetospheric environment have been studied in numerous papers.
With some care the terms “shear Alfvén wave” and “FMS” are applicable to the
actual MHD waves propagating in the magnetosphere since in most cases these
pure eigenmodes do not exist. However these two terms are extremely important
for understanding of wave processes in the planetary magnetosphere. The interested
reader is referred to the text by Glassmeier ( 1995 ) for a more complete review on
mode coupling in actual magnetosphere.
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