Geoscience Reference
In-Depth Information
Available candidates for a source of the ULF pulsations are the so-called firehose
and drift mirror plasma instabilities, which are driven by anisotropies of the plasma
pressure. We cannot come close to exploring these topics in any detail, but the
interested reader is referred to the text by Glassmeier (
1995
) for a more complete
treatise on magnetospheric plasma instabilities.
6.3.4
MHD Waves Propagating in Solar Wind
An indirect hint toward the existence of a variety of MHD waves, which can
propagate in the solar wind, has been provided by numerous observations and
has been supported by a number of theoretical studies (e.g., Kwok and Lee
1984
;
Takahashi et al.
1984
; Yumoto
1984
; Engebretson et al.
1987
). A wide variety of
the solar-wind-generated waves cover a wideband frequency range including ULF
pulsations region. It appears that such waves can cross the Earth's bow shock,
magnetosheath, magnetopause and then penetrate deep into the magnetosphere and
plasmasphere. There may also be an indirect way for the energy transfer from
the interplanetary space to the magnetosphere. For example, the Alfvén and FMS
wave energy can be transferred into the particle kinetic energy and back to the wave
energy via ionospheric interactions.
6.3.5
Reconstruction of the Magnetospheric Configuration
All the excitation mechanisms alluded to above share a common trait since they
are generated from the different kinds of plasma instabilities that can arise inside
the magnetosphere, at the magnetopause, or outside the magnetosphere in the solar
wind. In some sense, these mechanisms can serve as more or less permanent sources
of the ULF pulsations. In the next subsection we consider more impulsive sources
such as SSC and magnetic storm associated Pc5 pulsations and fast transients. The
SSC is due to the sudden changes of the solar wind flow followed by variations
of the dynamic pressure from the solar wind on the Earth's magnetosphere, that
in turn may be sufficient in order to change position of the dayside magnetopause.
The large-scale reconstruction of the magnetopause and the whole magnetosphere
results in the generation of ULF pulsations, which is believed to be almost
axisymmetric. The azimuthal wave numbers m associated with these pulsations
seem to be close to zero. This means that both main modes of the magnetospheric
eigenoscillations, i.e., toroidal and poloidal modes, can propagate through the
magnetosphere independently of each other. It appears that a number of ULF
pulsations are related to magnetospheric substorms in the magnetotail (Baumjohann
and Glassmeier
1984
).
Search WWH ::
Custom Search