Geoscience Reference
In-Depth Information
Pulsations are usually global in character. Certain simultaneous oscilla-
tions are traced with confidence over half of the globe. Oscillations observed
in one of the hemispheres are often visible in the opposite hemisphere ([3], [5],
[50], [54], [64], [84], [88], [114]). They have been discovered everywhere in the
magnetosphere, from its inner regions up to the magnetopause on the dayside
and up to distances of many tens of Earth radii in the magnetotail ([9],[13],
[18], [43], [44], [51], [63], [70], [102]). They were also observed in solar wind, as
well as in a shock wave that departed from the magnetosphere. We refer the
reader to the papers and monographs devoted to morphological properties
and physical aspects of generation of the geomagnetic ULF-variations (see,
e.g., [28], [30], [39], [90]). The monograph [89] covered the wide-ranging vol-
ume important aspects of ULF-waves, their main properties, comparisons of
ground and satellite observations within the magnetosphere, in the solar wind,
and in the magnetosphere tail, simultaneous occurrence at conjugate points,
tracing the oscillations from the ground through the ionosphere to the mag-
netosphere and the solar wind. Topics presented in this monograph include
numerical and experimental studies aimed at quantifying more precisely the
locations and mechanics of the ULF-waves.
The improving of theoretical and practical knowledge on nonlinear aspects
of ULF-oscillations was presented and exploited in [31].
3.2 The Physical Pattern
Magnetosphere
Let a certain source of MHD-waves with arbitrary polarization emerge in the
peripheral regions of the magnetosphere or in the solar wind. Radiation can
depart from the generation region as an Alfven wave or as a fast magnetosonic
wave (FMS). The Alfven wave will be guided by field-lines and propagate
directly from the generation region to the ionosphere.
The perturbations are carried inside the magnetosphere by FMS-waves
as well. Four regions are distinguished in the magnetosphere with regard to
FMS-wave propagation in it:
( a ) the propagation region;
( b ) the reflection region;
( c ) the damping region behind the reflection point and, lastly,
( d ) the region of resonance interaction.
The basic parameter determining the character of propagation is the
Alfven velocity c A = B 0 / (4 πρ 0 ) 1 / 2 ( B 0 is a magnitude of the geomagnetic
field, ρ 0 is the density of the background cold plasma). The geomagnetic field
and the magnetospheric plasma are inhomogeneous and therefore the Alfven
velocity distribution is significantly inhomogeneous as well. This leads to the
coupling of FMS- and Alfven waves.
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