Polarization patterns of Pi 2 magnetic pulsations and the substorm current wedge

The results of an analysis of 16 Pi 2 pulsations observed with an extended east–west chain of mid-latitude, ground-based magnetometers are reported. The events were chosen such that the center of the substorm current system, defined by using the midlatitude bays associated with these pulsations, was within the longitudinal extent of the station chain. A current wedge model is also used to interpret the observed polarization pattern of the Pi 2 pulsations. This pattern is used to locate the center of the Pi 2 current system. The centers of the Pi 2 and substorm current systems occur at the same meridian for only ∼65% of the events. This result suggests that the Pi 2 current system and the substorm current wedge are not always the same. The longitudinal extent of the Pi 2 current system estimated from the Pi 2 polarization pattern averages ∼90° or ∼6 hours local time for the events in the study. The sense of ellipticity of the waves, anticlockwise looking down the field line in the northern hemisphere, agrees with previous results from these latitudes. Estimates of the wave-phase difference between stations show that, in general, the eastern station of a station pair leads the western one at all local times for both H and D components. When plotted in a substorm coordinate system based on the mid-latitude bay, the phase difference per degree of longitude shows a tendency to decrease in the eastern portion of the current wedge. This longitudinal pattern of phase difference is consistent with the eastern, downward, field-aligned currents being less localized than the western, upward, field-aligned currents.

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