Space Plasma Seminar: Externally Driven Plasmaspheric ULF Waves Observed by the Van Allen Probes
Kazue Takahashi, Space Physics Group, Applied Physics Laboratory, Johns Hopkins University
Tuesday, April 1, 2014, 3:30-5:00pm
The Van Allen Probes, with their low-inclination elliptical orbits with apogee at 5.8 Re, are ideal for studying ULF waves in the plasmasphere. We analyze data acquired by these spacecraft on 8 November 2012 to gain new insight into quiet-time ULF waves in the plasmasphere. The waves exhibit strong spectral power in the 5-40 mHz band outside L = 3 and include muliharmonic toroidal waves visible up to the 11th harmonic, which is unprecedented in the plasmasphere. During this wave activity, the IMF cone angle was small, suggesting that the plasmaspheric waves were driven by broadband compressional ULF waves originating in the foreshock region. This scenario is supported by the tailward propagation of the compressional magnetic field perturbations that is determined using two-spacecraft phase delay analysis. The phase delay analysis, when applied to the azimuthal component of the magnetic field, also reveals signatures of field line resonance that have not been reported in space. Finally, using the observed toroidal wave frequencies, we estimate the plasma mass density for 2.4-5.8. By comparing the mass density with the electron number density that is estimated from the spectrum of plasma waves, we find that the ion composition is dominated by H+.
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