ROCSAT observation of the field line resonance effect in a plasma pulsation at topside ionosphere

During the recovery phase of the 29-31 October 2003 storm, a plasma pulsation event of large flow oscillation perpendicular to the field line was observed by ROCSAT-1 in a circular orbit at 600 km topside ionosphere. The flow undulation amplitude grows large as ROCSAT moves around the southernmost latitudes of the orbital coverage around the dusk region from dip latitude -50° to -53° to reach the largest observed oscillation amplitude of 380 m/s that is equivalent to an electric field of 18 mV/m. The undulation amplitude then decreases as ROCSAT moves back toward low latitudes in the northbound leg. A reversal of polarization and changes in polarization ellipticity are also observed as ROCSAT traverses across -50° dip latitude. The variation in wave amplitude together with the change in polarization suggests that a field line resonance (FLR) effect in the plasma pulsation has been observed along the ROCSAT orbit traversing different dip latitudes of magnetic shells. This event is different from past spacecraft observations of the Pi2 pulsation events manifested in the magnetospheric or plasmaspheric cavity resonance. The ROCSAT observation of the FLR effect will be for the first time to observe the plasma pulsation directly in space at a location just above the ionosphere where the induced ionospheric currents (or flows) have been manifesting the FLR effect observed on the ground. Our observation also indicates that a strong coupling exists between the global compressional eigenmodes and the excited transverse modes, as proposed in a model by Kivelson and Southwood [1986].