ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 64

Fig. 3.13. Spatial distribution of the total horizontal vector of Pi 2 oscillations
is shown by intensity contours. The locations of the stations are marked by dots.
Dashed line is the midnight meridian. The maximum at high latitudes is local, with
oscillation intensity in it about 15 nT

in the nighttime sector of the magnetosphere ([68], [75], [76]). It was discovered
the close coupling between the westward traveling surge (WTS) and Pi 2 pul-
sations during the substorm onset [71]. The oscillation intensity maximum
coincides with the equatorward electrojet boundary, i.e. the oscillation gener-
ation region coincides with the closed field-lines.
Virtually all the noted peculiarities can be explained by uniting two mecha-
nisms [72]. The first is the feedback instability [2], the other is the appearance
of an oscillatory regime as ionospheric conductivity changes sharply ([55],
[8]). Local spills into the ionosphere with a stationary electric field result in
a sharp increase of electron concentration and conductivity due to the first
mechanism. The background electric field is diminished as a result. The field
change will travel with Alfven velocity along the magnetic field-lines and,
being reflected from the conjugated ionosphere, can dampen Pi 2-type geo-
magnetic pulsations. The effectiveness of the mechanism is high enough, since
the background electric field in the surge can diminish by an order of mag-
nitude within a short time and reach 5

10 mV/m. The magnetic field in the
Alfven magnetospheric wave will then equal 10

−

200 nT.
The discovery of daytime Pi 2 demonstrates the possibility of oscilla-
tions penetrating into the low latitudes and into the vicinity of the equa-
tor through the ionosphere not only in the shape of spread-out currents,
and also testifies to the possibility of global cavity modes with very low az-
imuthal numbers [94]. One of the regions where resonance oscillations can
appear is the inner boundary of the plasma layer in the magnetosphere tail
[74].

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