Field line resonances and waveguide modes at low latitudes: 2. A model

Abstract

The power spectra of magnetometer data recorded at low latitudes show a remarkable similarity over the latitudinal range, 1.3<L<2.0. Power spectra from magnetometer sites along the east coast of Australia show a similar fine structure where adjacent power maxima are spaced between 3 and 5 mHz apart. The H component data also show a superposed feature of the power spectrum that varies with latitude according to that expected for field line resonance. In order to explain these data, the propagation of fast mode, ULF wave energy through a one‐dimensional model of the magnetosphere is examined. The model features an inner boundary at 1.1 RE, an input wave spectrum at the outer boundary (10 RE) that varies as 1/ƒ, and a realistic Alfven velocity that depends on the radial coordinate. The model includes a plasmapause, and in the inner plasmasphere, the Alfven velocity reaches a maximum at L= 1.6 then decreases as the larger plasma density of the upper ionosphere is approached. The azimuthal wave number and background magnetic field also depend on the radial coordinate, and a realistic variation of the field line resonant frequency is included. This model can reproduce important features of observed low‐latitude ULF power spectra and indicates that the interaction between waveguide and field line resonance modes is important in understanding low‐latitude data.