ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 61

magnetosphere. It was pointed out the similarity of the amplitudes and struc-
tures of wave packages in the H -component at stations in Iceland to the
longitudinal component detected by the CCE [96]. Spectral analysis showed
that Pc 3 pulsations have a peak at frequency f 60 mHz at CCE. This fre-

quency is close to the estimate by [101]. A peak on the same frequency was
also discovered at the magnetometer points.
At the same time at the higher latitude station ( Φ =78 . 9 ◦ N) and at the
conjugate station located in the Southern Hemisphere, the pulsation ampli-
tude in the Pc 3 range is considerably smaller. The existence of a high-latitude
maximum in the Pc 3 range at geomagnetic latitudes Φ

∼

70 ◦ and a
sharp fall in pulsation amplitude with northward movement are well known
facts [38].
Takahashi et al [96] relate the intensity maximum at Φ

65 ◦ −

≈

70 ◦ by an Alfven
wave passing along the magnetopause to the cusp boundary towards Earth.
The high-latitude station of Φ =78 . 9 ◦ is then to the North of the cusp. This
is why the lack of oscillations in this region can serve as an indirect indication
to the most probable path of the Alfven wave into the high-latitude region—
from the generation area within the upstream region to the magnetosphere,
through the magnetopause, by transformation of FMS into an Alfven wave
and finally propagation along the external force tubes on the inner boundary
of the cusp towards the ionosphere and the Earth.
We believe that the transformation of FMS- into Alfven perturbations
does not occur on the high harmonics of standing Alfven oscillation, but on
Alfven waves traveling towards the Northern Ionosphere. With this excitation
mechanism a natural explanation is found for the fact of asymmetry in pulsa-
tions recorded at conjugated stations. At the station in the Southern Hemi-
sphere, the pulsation amplitude is considerably smaller than at the stations
conjugated with it. This difference was explained in [96] by the asymmetry of
ionospheres in the Northern and Southern Hemispheres.
Pc 3 pulsations were observed in the very high latitude ionosphere, in the
so-called ionospheric cusp, in the magnetosheath and on the ground [58].
The data used in the ionospheric analysis were obtained by the HF radars
of SuperDarn and near the magnetopause by the Geotail satellite.
If the initial wave packet of FMS-waves is supposed to propagate north-
wards and the condition of wave synchronism with traveling Alfven waves
is fulfilled, then the excited Alfven wave package will also move towards
the Northern Hemisphere. It will be demonstrated in Chapter 7 that dur-
ing transition from day to night, the magnetosphere wave resistance is
close to the ionosphere resistance and the Alfven wave reflection coecient
proves to be small. It follows from estimates that it is of the order of the
ratio of H -component amplitudes at conjugate stations. That may mean
that the Alfven package, approaching first the Northern Ionosphere, was
weakly reflected from it and left for the Southern Hemisphere with a small
amplitude.

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