ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 55

The allocated latitude band where the spectral peaks are localized was
interpreted as excitation of FLR by cavity or waveguide mode. From these
frequencies it is possible to make a rough estimate of the Alfven velocity as
c A ≈ 40 km/s. Such velocity requires too large hydrogen concentrations as

compared to those found by satellite measurements. These results may point
to a noticeable presence of O + ions near the magnetopause [69].
The results of ground, radar and satellite measurements can also be coordi-
nated by assuming that FMS-wave reflection occurs not on the magnetopause,
but on the bow shock, i.e. in a region of very low Alfven velocity [33].
There is another possibility to account for the observed low frequencies
of Pc 5 pulsations in the framework of the cavity resonance mechanism. The
frequency of the lowest cavity mode can be determined not by oscillations
between the turning point and the magnetopause (or bow shock), but by
conditions in the magnetosphere tail or on its boundary. Satellite observations
have shown that compressional Pc 5 appear mainly on the nightside on L>
7

9 min are constantly present in
the magnetosphere tail as well at distances of 10

− 8 . Such standing waves with periods of 3 −

− 22R E ([1], [82], [83]).

Traveling Magnetospheric Twin-Vortices

Twin-vortices ionospheric current systems in the Pc 5 range moving tailward
at the velocity of 2

5 km/s were found in ([19], [22]). These systems were
attributed to local inhomogeneities of the solar wind leading to changes in
the dynamic pressure on the magnetopause and in longitudinal currents. The
characteristic spatial scale of the vortices at ionospheric altitudes is

−

≈ 2000 km.

They are transmitted towards the ionosphere by an Alfven wave.
The large changes in the solar wind dynamic pressure in the shape of
an unipolar pressure jump (see Fig. 3.7(a)) produce a Field-Aligned Current
(FAC) proportional to the derivative of the pressure perturbations with
respect to the azimuthal coordinate [22]. The signs of the derivative is op-
posite on the two sides of the pressure pulse, i.e. the excited FACs are of
bipolar character. The stepwise pressure change is also sucient for the exci-
tation of a bipolar system (Fig. 3.7b) [45]. In this case, the FAC is proportional
to the second derivative of the pressure perturbation.
A couple upward and downward FAC spaced along the latitude at about
2000 km are closed in the ionosphere by Pedersen currents. The ground mag-
netic field of the ionospheric currents under the polar homogeneous ionosphere
is solely due to the Hall current system ([20], [21]) that is the bipolar Hall
current vortices produce vortices in the ground magnetic field.
Figure 3.8 (b) present NS -and EW -components from the Scandina-
vian magnetometer array of stations (Fig. 3.8(a)). The main peculiarity of
the records is the isolated NS -component bipolar pulse as well as a unipo-
lar pulse in the EW -component. There are no noticeable variations in the
vertical component. Figure 3.8 clearly shows earlier onsets of variations at