Longitudinal structure in Pi 2 pulsations and the substorm current wedge

The characteristics of mid‐latitude Pi 2 pulsations are studied using a subauroral east‐west chain of magnetometers which spans over 4 hours local magnetic time. The mid‐latitude magnetic bays associated with the Pi 2 pulsations are used to define a longitudinal coordinate system based on the substorm current wedge model. The wave properties, frequency, polarization, and east‐west phase variations are examined in this coordinate system. This allows us to compare the wave characteristics at longitudes between the meridians of the two field‐aligned currents of the substorm current wedge with the characteristics both east and west of these meridians. In an attempt to minimize the influence of preexisting current systems on our estimate of the substorm current location we use only Pi 2 pulsations which follow a magnetically quiet interval and which start simultaneously with a magnetic bay. We find that the longitudinal pattern of the horizontal polarization ellipse azimuth found earlier extends beyond the meridians of the field‐aligned currents. The sense of wave horizontal polarization is predominantly counterclockwise at all longitudes, though the incidence of linear and clockwise polarization increases with distance from the field‐aligned current meridians. Estimates of signal phase differences between station pairs, if interpreted as azimuthal phase propagation, show that westward propagation dominates west of and within the field‐aligned current meridians but the eastward propagation dominates east of the current system. This latter observation fits a recent model introduced to explain the longitudinal variation in polarization azimuth. We also looked for variations in the signal frequency among stations and found some variation in about half the events studied but could find no systematic behavior.