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Fig. 6.9
An example of magnetic field variations during an isolated magnetospheric substorm
(
upper panel
) and a Pi2 pulsation (
bottom panel
). The data displayed in the
bottom panel
are high
pass filtered time series of the upper record. Taken from Glassmeier (
1995
)
21:57 UT is an evident signature of the substorm onset. The data at the bottom panel
is high pass filtered to yield a magnetogram of the Pi2 pulsation with period about
150 s and amplitude about 10 nT. The trains of Pi2 pulsations are usually observed
at the nightside of the Earth. Their amplitude about 1-50nT tends to maximize in
the vicinity of aurora region at midnight.
The long-period pulsation trains, such as Pc3, Pc4, Pc5, and Pi2 pulsations, are
believed to be due to eigenoscillations of the Earth magnetosphere (Kato
1962
;
Zibyn and Yu
1965
). The Alfvén oscillations can explain the basic properties of
the Pc5 pulsations and, in some cases, the patterns of the Pc4 pulsations. The
observed dependence of pulsation period on the latitude is consistent with that
predicted by the FLR theory we have treated in the previous sections (Guglielmi
and Troitskaya
1973
). Likewise, a number of pulsation events exhibit a rather
localized wavefield of extension 100-200 km in north-south direction and about
500-1;000 km in east-west direction (Glassmeier
1980
). The additional argument
that is in favor of the resonance origin of the Pc4, Pc5 pulsations is that these
pulsations are well correlated at the magneto-conjugate points (Guglielmi and
Troitskaya
1973
). Lanzerotti and Fukunishi (
1974
) have found that in the ground
magnetic observation the odd mode of the Alfvén oscillations is prevailed so that the
amplitude of the oscillation reaches a peak value at the equator. On the other hand
the amplitude of these pulsations grows with increase of the latitude (Ziesolleck
et al.
1993
). This suggests that the source of the pulsations is at the periphery of
the magnetosphere. The fundamental mode of the FLR oscillations manifests itself
through Pc5 pulsations as observed on board the satellites OGO 5 and GEOS 2
(Singer and Kivelson
1979
; Junginger et al.
1984
). A signature of the fundamental
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