Analysis of geomagnetic sudden impulses at low latitudes

We examine the characteristics of a train of ULF waves observed in the magnetosphere and at ground after the sudden impulse (SI) onset of 8 June 2000. A highly monochromatic and large-amplitude wave at f = 3.3 mHz was observed between 9:10 and 9:50 UT at the GOES8 orbit (on the dawn flank of the magnetosphere) and across a wide longitudinal range of ground stations from low to high latitudes. The combination of the long period of these pulsations (5min), their extended duration (∼40min), the latitude-independent frequency, and the small azimuthal wave number suggests the occurrence of a global magnetospheric mode driven by the sudden enhancements of the solar wind dynamic pressure. The amplitude and cross-phase analysis of the wave activity on the ground and the polarization pattern indicate that the global mode coupled to the field line resonance (FLR) occurs at different latitudes at different local times. Such FLRs occurred at latitudes smaller than usually observed at the same frequency, suggesting a significant reduction of the local field line eigenfrequencies as compared with usual values. A model estimation (time of flight approximation) of such eigenfrequencies suggests a change in the magnetospheric field geometry characterized by more elongated field lines than those for usual conditions. It was probably caused by the compression of the magnetosphere driven by the pressure pulse coupled with the stable northward orientation of the interplanetary magnetic field.