The ionospheric signature of Pi 2 pulsations observed by STARE

This study extends the work of Sutcliffe and Nielsen (1990) in which a classical Pi 2 pulsation was first isolated in Scandanavian Twin Auroral Radar Experiment (STARE) data. A high‐pass‐filtering technique is used to remove the background electric field in the STARE data and so reveal the spatial and temporal ionospheric signatures of the Pi 2 pulsation electric fields. A number of events are identified and examples presented in which pulsation electric fields up to 50 mV/m are observed. Magnetic field oscillations computed from the filtered STARE data using the Biot‐Savart law correlate well with pulsation magnetometer data. A 180° phase difference is observed between high‐ and low‐latitude X component pulsations. The ionospheric signature of a Pi 2 is located slightly poleward of the core of the auroral breakup region where the southward, westward, and northward directed background electric fields converge; the strongest pulsation fields occur in the region of equatorward directed electric fields. The ionospheric electric field patterns of the Pi 2 pulsations determined from the STARE data correlate well with those modeled for a transverse Alfvén wave incident on an east‐west aligned high‐conductivity strip in the ionosphere.