Polarization changes of geomagnetic Pi 2 pulsations associated with the plasmapause

The polarization characteristics of substorm-associated Pi 2 pulsations are studied in detail on statistical and nonstatistical bases by using magnetic data from a network of stations (L = 3.2–4.4) along a geomagnetic meridian and a conjugate station (L ∼ 4) in the southern hemisphere. The predominant periods in the Pi 2 wave trains are usually constant at all latitudes, whereas the polarization characteristics of the predominant oscillations change drastically among the stations. That is, reversals of the sense of polarization between two stations were often observed. Such polarization reversals result from the latitude dependence of the wave phase: the D (azimuthal) pulsation components oscillate in phase at all stations, whereas the oscillations in the H (north-south) components have large phase shifts among the stations. The H component phase shift is often 180° between Durham (L = 3.2) and Lac Rebours (L = 4.0). A satellite measurement of the plasmapause location during one Pi 2 event showed that the large H component phase shift occurred near the plasmapause. Therefore the Pi 2 oscillations, which are originally excited at high latitudes in the magnetosphere, apparently couple to the shear Alfven waves of the resonant local field lines at the plasmapause because of the sharp decrease in the wave phase velocity just inside the plasmapause. This result suggests the possibility of determining the location of the night side plasmapause by monitoring the polarization changes of Pi 2 pulsations along a geomagnetic meridian. The Pi 2 data from the conjugate pair stations showed that the H components oscillate in phase, whereas the D components oscillate out of phase. Therefore it is concluded that the Pi 2 waves observed near L = 4 can be characterized as odd mode standing Alfven waves.