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We report multisatellite observations of the oscillations in the subauroral polarization stream (SAPS) during a severe magnetic storm on 20 November 2003. The SAPS oscillations (SAPSOs) occurred during the main phase of the magnetic storm when the y component of the southward interplanetary magnetic field (IMF B-Y) turned from positive to negative. The SAPSOs were first observed in the premidnight sector and propagated toward the dusk sector. The formation and evolution of SAPSO corresponded well with the plasma sheet ions injection and precipitation, indicating that the SAPSOs are possibly generated by the interaction between the hot plasma sheet and the cold plasmasphere under particular conditions (e.g., change of the polarity of IMF B-Y accompanied with a sudden enhancement of plasma sheet ion density). The hemispheric asymmetry of the SAPS channels is suggested to be related to the hemispheric differences in the ionospheric plasma condition and the ionospheric convection.

Plain Language Summary Subauroral polarization streams (SAPS) are narrow channels of strong westward ion flows in both the subauroral ionosphere and the conjugated inner magnetosphere on the nightside during geomagnetic storms and substorms. SAPSs provide important clues on magnetosphere-ionosphere coupling. In this letter, we report SAPS oscillations (SAPSOs) during the severe geomagnetic storm on 20 November 2003 using observations from multiple Defense Meteorological Satellite Program (DMSP) satellites. Our results indicate that the SAPSO possibly result from the sunward flow of dense plasma sheet ions and the waves generated by the interaction between the cold plasmaspheric plasma and the hot plasma sheet ions.