FINE-STRUCTURE IN THE SPECTRA OF LOW-LATITUDE FIELD LINE RESONANCES

The power spectra of low latitude (L<3), groundbased recordings of day side geomagnetic pulsations in the 10-100 mHz band indicate that these pulsations come from discrete, driven field line resonances which are coupled to compressional, magnetohydrodynamic, cavity or waveguide mo des. The spectral bands of individual harmonics of the field line reso nances seen by the magnetometers have envelopes which are-typically 10 's of mHz wide but also show a fine scale structure of almost equispac ed peaks with separations of about 3 to 5 mHz. A WKB analysis of a cav ity or waveguide model shows that the fine structure is likely due to field line resonances driven by waveguide modes with turning points wi thin the plasmasphere and reflection at the dayside magnetopause.