Poloidal Mode Field Line Oscillations in the Inner Magnetosphere

Plasmaspheric flux tubes may execute toroidal and poloidal standing oscillations when suitably stimulated by incoming ULF waves. The flux tubes can also support the field-aligned propagation of VLF whistler mode waves under appropriate conditions. The VLF signals may exhibit small periodic Doppler shifts due to radial flux tube motions. We examined about 100 such ULF/VLF Doppler events recorded over one year at L=2.5 using artificially generated whistler-mode VLF signals and ground magnetometers. Joint peaks in the VLF Doppler spectra and ground magnetometer spectra occurred at frequencies corresponding to the ion- cyclotron instability in the upstream solar wind. The VLF Doppler shifts were most likely due to radial motion of flux tubes of a few kilometers, driven by the east-west electric field of propagating ULF waves. When the frequencies match, these waves may couple to standing poloidal and toroidal field line oscillations, producing resonance signatures in both the H and D components on ground magnetometers.