PROBLEMS OF GEOCOSMOS

Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)
4 Discussion
Very many works are devoted to studying Pc5 geomagnetic pulsations; however, the nature and spatial
features of these oscillations have been studied insufficiently. In many respects this is related to the fact that
the class of Pc5 pulsations combines different types of oscillations with periods in the same range but with
different generation origin. In contrast to other types of geomagnetic pulsations, Pc5 oscillations are
characterized by not only large periods but also by huge amplitudes (~30-100 nT), reaching 300–600 nT in
the recovery phase of the superstorms [e.g., Kleimenova and Kozyreva, 2005].
It is generally accepted that the Kelvin–Helmholtz instability at the magnetopause or in the entrance layers of
the magnetosphere is the main source of Pc5 pulsations. In addition to field line resonance, Pc5 pulsations in
the magnetosphere can be also generated due to the development of the drift–mirror instability of the ring
current under the conditions of large β (see [Pilipenko, 1990] and references therein). These waves have the
large azimuthal numbers (m ~ 50–100). They are mainly registered on satellites and as a rule do not observed
on the ground. Generation of the global magnetospheric cavity mode in the Earth’s magnetosphere can be
one more source of Pc5 pulsations [e.g., Kivelson et al., 1984]. In this case the poloidal oscillations with a
considerable compression component in the radial direction originate in the magnetosphere. Such oscillations
were often observed on geostationary satellites in the post noon sector of the Earth’s [e.g., Hudson et al.,
2004]. Oscillations can be also a result of a direct penetration of the waves from the solar wind [e.g., Kepko
et al., 2002]. Thus, ULF pulsations can be resulted from the simultaneous action of different sources.
We found (Fig. 3) that during the storm main phase the values of the ULF-index in the IMF is more than once
smaller than in the magnetosphere (GOES data) and the estimated correlation coefficient between the ULFindex
in the IMF and in the magnetosphere is 0.73 in the case of the strong magnetic storms and 0.67 in the
case of the moderate storms. This suggests that the most part of ULF waves, observed on the ground, are
generated inside of the magnetosphere and only a small part of the wave turbulence penetrates from the solar
wind. The correlation coefficient between the ULF-index on the ground and in the magnetosphere is 0.96 in
the case of the strong magnetic storms and 0.91 in the case of the moderate storms. By this it means that
practically all Pc5 pulsations, observed on the ground at the auroral latitudes, are exited by a magnetosphere
origin.
We have applied the superposed
epoch technique to analyzing the
IMF and solar wind parameters
variations of all selected storms.
The results of this analysis are
presented in Fig.4. In the initial
phase of a magnetic storm there is
observed a strong enhancement in
variations of the solar wind
dynamic pressure (Fig. 4) and in
the wave turbulence in the IMF
according to the estimated values
of the ULF-index (IMF), as it is
seen in Fig. 3a. In a storm initial
phase, the ULF-index (IMF)
maximum is much stronger than
during a storm main phase. This
effect is not so clearly marked in
the ground (Fig. 2) and in the
GOES (Fig. 3) ULF-index
Fig. 4 The distribution of the solar wind dynamic pressure
interplanetary, Bz component IMF and Dst index during 19
strong magnetic storms (left panel) and 37 moderate
magnetic storms (right panel).
variations. Only a gradually
increasing of the ULF-index
values, preceded the storm main
phase maximum, can be noticed in
a storm initial phase. We have to
keep in mind that the ground
ULF-index is calculated for
selected latitude range. In our case the ULF-index is computed for the auroral latitudes (Φ = 60°–70°).
However, it is well established [e.g., Kozyreva et al., 2004; Kozyreva and Kleimenova, 2004, 2007] that in
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