Eastward propagation of transient field-aligned currents and Pi 2 pulsations at auroral latitudes

Transient field-aligned currents associated with the substorm expansive phase produce the damped, quasisinusoidal, geomagnetic pulsations called Pi 2's. Mid-latitude and auroral latitude measurements of the fields of these pulsations have indicated that they are produced, for the most part, by westward propagating field-aligned currents at auroral latitudes, although some studies have shown hints of eastward propagation, particularly far to the east of the longitude of the onset of the expansive phase. This study uses data from the University of Alberta magnetometer array to identify the characteristics of the eastward propagating mode at high latitudes by measuring polarizations and apparent group and phase velocities in regions far to the east of the expansive phase. The results show that eastward propagating Pi 2's have very high phase velocities (typically 40 km/s) which are comparable with those of the westward propagating mode. The polarizations and field configurations are compatible with a field-aligned and ionospheric current model which is very similar to that for the westward propagating component. These current models suggest that Pi 2's may be caused by shear Alfven waves produced by an azimuthally expanding, fast mode wave. The shear Alfven waves are reflected from the auroral ionosphere to produce the latitudinally localized field-aligned and ionospheric currents in the eastward (and westward) propagating modes