ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 51

Ionosphere

The described path for the wave propagation from the magnetopause to inner
L-shells is not the only one possible. The waves that have reached the high-
latitude ionosphere can cause perturbations propagating along the ionosphere
to the middle and low latitudes. The ULF-waves in the ionosphere possess a
tremendous skin depth scale many times larger than the characteristic vertical
scale of the ionosphere.
There is an essential difference between the mechanisms for ionospheric
transformation of an Alfven wave and an FMS-wave. The Alfven wave brings
a longitudinal current to the ionosphere, which does not penetrate into the
atmosphere because of its insignificant conductivity and spreads over the
ionosphere. The total current system of the reflected and incident waves is
arranged so that the magnetic effect from it under the ionosphere vanishes.
Only the magnetic effect of the Hall currents can be observed, while above
the ionosphere, the magnetic field is determined by a system of longitudinal
currents closed in the ionosphere by the Pedersen current [20]. For this reason
the magnetic components under the ionosphere must be turned by π/ 2ifthe
source is an Alfven wave. The ground signature of the Alfven wave incident
onto the ionosphere manifests itself in the FMS-polarization. It is important to
note that these concepts can also be extended to the wide range of oscillation
periods from several seconds to several days.
On the contrary, the ionosphere is almost transparent for the FMS-mode
causing two electrical currents - ionospheric and terrestrial. The relative con-
tribution of both depends on a number of factors, but primarily on the ratio
of the ionospheric to the Earth's conductivities.

3.3 ULF-Waves on the Ground and in Space

Pc5 Pulsations

Pc 5 pulsations are basically a high-latitude phenomenon. They are character-
ized not only by long periods (150 − 600 s), but by tremendous amplitudes as

well. Their amplitudes are 100 nT at high latitudes, and under condi-

tions of high activity, it can rise to 400

≈ 40 −

600 nT.
Pc 5 are usually observed at latitudes of 60

−

75 ◦ . Figure 3.3 [42] shows the
latitudinal dependency of the Fourier amplitudes and relative phases. The
curves illustrate the characteristics typical for field line resonances (FLRs)
([10], [90]). The latitude of maximum amplitude decreases with increasing fre-
quency. For all frequencies, the phases change with

−

π near the corresponding
amplitude maxima, which also is a typical FLR feature.
The oscillations occur more often in the morning and afternoon hours. For
strong geomagnetic activity ( Kp

≈

5) excitation of Pc 5 oscillations is about
equally probable in the interval from 6 to 18LT [3].

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