Spectral characteristics of magnetic Pi 2 pulsations in the auroral region and lower latitudes

The spectral characteristics of magnetic Pi 2 pulsations are studied in detail using data obtained simultaneously over a wide latitudinal range from the auroral region ( L =7.5) through low latitudes ( L =1.8). Pi 2 is revealed to be observed almost simultaneously with the onset of a substorm expansion over the wide latitudinal range with the common predominant period. The Pi 2 amplitude shows a primary maximum at the auroral oval and an additional secondary maximum near the plasmapause. The Pi 2 period generally becomes shorter (longer) when the associated auroral breakup starts at lower (higher) latitudes. Pi 2 is also observed simultaneously at a pair of conjugate stations in the auroral region with similar waveforms. These observational results support the torsional oscillation model which suggests that Pi 2 is caused by the hydromagnetic torsional oscillation of the geomagnetic field line anchored on the auroral oval.