Initial deflection of middle-latitude Pi2 pulsations in the premidnight sector: Remote detection of oscillatory upward field-aligned current at substorm onset

In this study, we examined middle- and low-latitude Pi2 events to address the following two issues regarding the well-known substorm current wedge (SCW) model for Pi2 pulsations: (1) the center of the SCW, which is estimated using the Pi2 polarization pattern, is not always collocated with that determined using the magnetic bay pattern; and (2) although ideally Pi2 hodograms would be linear, they tend to become circular. In this study, auroral breakup events were identified from Polar Ultraviolet Imager data. We assumed that the ionospheric footprint of the upward field-aligned current (FAC) in each event was located at the position of the auroral breakup and subsequently calculated the signature of the magnetic variation at the middle-latitude station Zyryanka (ZYK; GMLAT = 59.6°) that was generated by the upward FAC. In order to examine the magnetic effects of the upward FAC, we selected Pi2 events that were observed when ZYK was located on the duskward side of the auroral breakup location. A total of 112 events were selected and analyzed in this study. It was found that the location of the upward FAC of the SCW could be estimated more accurately by using an azimuth value predicted based on the initial deflection of the middle-latitude Pi2. Our results suggest that the circular shapes of Pi2 polarization curves are caused by the delayed driven Alfvénic waves that are superimposed on the geomagnetic northward components of SCW oscillations.