Simultaneous ground-based and satellite observations of Pc5 geomagnetic pulsations: A case study using multipoint measurements

Pc5 pulsations with a latitude-independent frequency of ∼2.8 mHz (period ∼6 min) were observed with ground-based magnetometers in the morning sector (0700–1000 local time) from 0730 to 0810 UT, April 29, 2001. The pulsations exhibited an amplitude peak and a ∼180° phase change at ∼67° geomagnetic latitude. A vortex structure of the equivalent ionospheric current system was also observed centered at the geomagnetic latitude between 67° and 71°. During the Pc5 event, the Polar spacecraft was located near the morningside magnetic equator and crossed magnetic field lines conjugate to the ground stations at which the pulsations were strong. Polar observed ∼2.8 mHz pulsations in the radial electric field and compressional magnetic field components. Since toroidal mode Alfvén waves in the magnetosphere are characterized by an electric field perturbation in the radial direction, the simultaneous presence of pulsations in both components indicates that a field line resonance (FLR) was driven by compressional pulsations. The ground H component at the station conjugate to Polar and the radial electric field perturbation at Polar oscillated with a great similarity without a phase delay. From an analysis of the ground-satellite data, we suggest that the ground perturbations are associated with toroidal mode Alfvén waves rather than compressional mode propagating across magnetic field lines. In addition, we discuss the source of the compressional perturbations observed at Polar using solar wind and geosynchronous data.