Pc 5 ULF waves observed simultaneously by GOES 7 and the CANOPUS magnetometer array

Abstract

Applying pure state filtering techniques to 1 month (March 1990) of GOES 7 and CANOPUS (Canadian Auroral Network for the OPEN Program Unified Study) magnetometer data, we have detected several dominantly compressional Pc 5 events in the satellite data which were well correlated with the pulsation signal recorded by the ground stations. On the ground, near the GOES 7 magnetic footprint, these pulsations were seen with polarization characteristics similar to those observed at geosynchronous orbit, i.e., without ionospheric rotation of the transverse polarization major axis. Using data from 13 stations of the CANOPUS array, we show that these compressional wave events seen by GOES 1 at L = 6.67 were associated with field line resonances (FLRs) at higher L values (L ≥ ∼8), propagating antisunward with small azimuthal wave numbers. Besides these compressional waves, we identified a number of azimuthally (east–west) polarized transverse Pc 5 events at geostationary orbit. Near the subsatellite point these appeared on the ground with the polarization major axis aligned in about the north–south direction, i.e., rotated through ∼90° between magnetosphere and ground. The spatial characteristics, as derived from the ground array data, show that these transverse events represent FLRs on or near the satellite's L shell. By means of selected events with clear FLR characteristics in the CANOPUS data, we determine the resonance position from the ground array data and examine the variation of the signal characteristics at geosynchronous orbit with increasing radial distance of GOES 7 from the resonant field line. We find that the transverse wave characteristics, dominant near the resonance, decay on relatively small‐scale lengths and are transformed into compressional wave characteristics within about 1–2 RE from the resonance position. The diurnal variation of the signal parameters for these events suggests that compressional Pc 5 waves at geosynchronous orbit associated with FLRs at L > 8 are preferentially generated near local noon. Finally, via consideration of solutions of the uncoupled toroidal mode wave equations based on the observed signal frequencies and resonance positions, we discuss the field‐aligned harmonic of these resonances.