ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 45

3
ULF-Waves on the Ground and in Space
3.1 Introduction
Geomagnetic ULF-oscillations are excited by solar wind on the magne-
tospheric boundary where the wind's velocity vanishes ([15], [16], [17]). The
waves arise due to the discontinuity of the tangential velocity (Kelvin-
Helmholtz instability). The mechanism for the build-up of oscillation resem-
bles the onset of ripples on a water surface, in a field of corn or of flags
fluttering in gusts of wind. Oscillations evolve mainly on the flanks of the
magnetosphere. Like any surface waves, they decay as they penetrate deeper
into the magnetosphere. Perturbations in the Alfven mode are guided and
propagate with practically no distortions from the magnetosphere boundary
to the high-latitude ionosphere.
The peculiarities of wave generation and its relation to the morning and
evening sectors of the magnetosphere must manifest in the different pulsation
characteristics. In the daily variation of long-period oscillation frequency de-
pendency, two anomalies ought be observed, one in the morning and one in
the evening. In addition the horizontal vector is expected to change is rotation
direction at these times as well.
An intensity maximum must exist at high latitudes in the region of mag-
netopause projection on the Earth's surface.
Starting with the 1st International Geophysical Year, the MHD-wave na-
ture was confirmed experimentally both in observatory explorations over many
years of regular observations and during specially arranged experiments in
synchronous observations of oscillations by meridional and latitudinal chains
of magnetometer stations ([3], [4], [12], [18], [36], [46], [14], [56], [57]).
In addition to large-scale hydrodynamic instabilities within the magne-
tosphere and on its boundary, pulsations can also be caused by the kinetic
micro-instabilities of magnetospheric plasma and/or of the solar wind ([9], [12],
[25], [36], [108]). Pulsations can also be excited by solar wind inhomogeneities
reaching the magnetosphere. Lastly, the ionosphere is another possible region
for the emergence of geomagnetic pulsations.