ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 63

Pi 2 Pulsations

Irregular pulsations of the Pi type are closely linked to a magnetospheric sub-
storm [66]. Pulsations of this type have the characteristic shape of a damping
oscillation train with periods of 60 10 min. The

most important feature of Pi 2 is the onset of these oscillations in the explosive
phase of the substorm. An abrupt Pi 2 splash is a kind of signal for the onset
of the explosive phase. A sequence of Pi 2 can be observed in the course of
substorm evolution. The appearance of beam-shaped auroras is characteristic
of the generation period of these oscillations, which testifies to the injection
of electron flows. With increasing brightness of the auroras, their amplitudes
grow as well. However, at a certain value of their brightness, their amplitudes
begin to decrease and a kind of oscillation quenching takes place.
These oscillations are a typical nighttime phenomenon with a clear maxi-
mum near the auroral zone (63

− 100 s and a duration of 5 −

70 ◦ ) and a comparatively small intensification
near the plasmapause ([32], [38], [44]). On the basis of these facts, it was as-
sumed in earlier works that the primary excitation process takes place on the
auroral magnetic field-lines, while the plasma density gradient on the plasma-
pause only facilitates the secondary resonance oscillation enhancement in this
region.
The oscillations embrace at least 1 / 3 of the globe. Their intensity maxi-
mum occurs approximately in the midnight hours. Comparison of data shows
that oscillations appear virtually simultaneously at points several thousand
kilometers apart. Phase delays between identically designated components at
a middle-latitude ( Φ

−

50 ◦ ) station chain stretching along the geomagnetic
parallel for about 60 ◦ are presented for two individual cases in Fig. 3.13. The
azimuthal wave number for Pi 2 oscillations changed from 0 to 5. The charac-
teristic phase velocity along the parallel is within the first hundreds of km/s.
The major axis of the ellipse is directed towards the intensity maximum region
at high latitudes [6].
Contours in Fig. 3.13 demonstrate the intensity distribution of the total
horizontal vector of Pi 2 oscillations. The positions of the recording stations are
marked by dots. Intensity increases monotonically towards the high latitudes
and the midnight meridian (dashed line). The isolines stretch approximately
along parallels, embracing virtually all the Earth's nightside. The maximum
at high latitudes is local, with oscillation intensity in it about 15 nT.
The influence of major ionosphere conductivity anomalies on Pi -
oscillations was revealed by Shimizu and Yanagihara [85] who discovered the
intensification of nighttime high-latitude Pi -oscillations on the dayside in the
equatorial electrojet region. The result was confirmed in ([63], [94]) and it was
shown that these oscillations are constantly present at low latitudes and near
the equator ( L< 2).
One of the hypotheses about the excitation of Pi -oscillation is the release
of magnetic field energy in the magnetosphere tail in the course of magnetic
reconnection processes with subsequent resonance excitation of the field-lines

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