Magnetospheric MHD Resonances and ULF Pulsations - Ultra and Extremely Low Frequency Electromagnetic Fields - page 228

the Hall current j H D H . O z E / is parallel to x axis. The Pedersen and the Hall
conductivities of the E layer are assumed to be constant.
Thus the field-aligned currents of the incident and reflected Alfvén waves and the
ionospheric sheet system of currents, shown in Fig. 6.6 , can be split into poloidal
and toroidal current systems. The first one includes the field-aligned currents and
the ionospheric Pedersen currents. In such a case the magnetic perturbation, ı B p ,
are concentrated inside the poloidal current system so that the magnetic effect is
undetectable below the ionosphere. In general, formal proof of this assertion can
be found in McHenry and Clauer ( 1987 ). The toroidal system builds up as a result
of the Hall currents flowing in the E layer of the ionosphere. In our model these
currents are closed in the infinity .x !˙1 /. The magnetic field of the toroidal
currents, ı B t , is perpendicular to ı B p . This means that the magnetic perturbations
in the atmosphere are perpendicular to the magnetospheric field of the Alfvén wave.
The ionosphere therefore changes the wave polarization by 90 ı . In our model
we have ignored the field line curvature and the inhomogeneous distribution of
the Pedersen and Hall conductivities in the ionosphere. Actually the ionosphere
produces a rotation of the polarization plane in the angle range from 0 ı to 90 ı .
The detailed calculations of this problem are found in numerous papers (e.g., see
review by Glassmeier 1995 for details). Notice that the latitude variations of the
ULF pulsation period, as observed in space and on the ground, are in favor of
the ionospheric rotation effect. In particular the latitude dependence of the Alfvén
resonance oscillations in space is detected in azimuthal component (D component),
whereas the ground-based observation exhibits the same dependence in meridional
field (H component) that is consistent with the rotation of components by the
angle 90 ı .

6.3 Sources of ULF Pulsations

6.3.1 Observations of ULF Pulsations

Observations and study of ULF MHD waves is certainly necessary as they transmit
energy, momentum, and most importantly they provide us with information about
magnetospheric dynamics. A variety of these waves occurring in the magnetosphere
and ionosphere result in the generation of ULF geomagnetic pulsations that
have been identified in both ground-based and satellite observations. Periods and
frequencies of the ULF pulsation vary from 0:2 to 600 s, and from several milliHertz
to several Hertz, respectively. Below is the frequency range of magnetic storms. The
amplitudes of the ULF pulsation typically change from 0:1 to 50 nT.
In standard geophysical practice the ULF pulsations are classified according to
their period. They can be also divided into two classes depending on whether the
pulsation accompany substorms or not (e.g., see Jacobs 1970 ; Nishida 1978 ). The
latter class includes the regular quasiharmonic oscillations, which are termed Pc
oscillations (Pulsations continuous). This class of the ULF pulsations can be in turn