DISPERSIVE FIELD LINE RESONANCES ON AURORAL FIELD LINES

The formation of dispersive Alfven resonance layers is investigated us ing a three-dimensional, two-fluid, magnetically incompressible model, including electron inertia and finite pressure. The equations are sol ved in ''box'' geometry with uniform magnetic field bounded by perfect ly conducting ionospheres. Field line resonance (FLR) is stimulated wi thin a density boundary layer with gradient transverse to ambient B; a parallel gradient in the Alfven speed is also included. Numerical res ults show that the resonance amplitude is largest on the magnetic shel l with eigenfrequency matching the frequency of the surface wave propa gating on the density boundary layer. Efficient coupling between the r esonant Alfven wave and surface wave produces a relatively narrow FLR spectrum, even when the driver is broadbanded. Effective coupling to t he external driver occurs only for long-wavelength azimuthal modes. It is shown that the parallel inhomogeneity limits radiation of dispersi ve Alfven waves by the FLR. The results provide new insights into low- altitude satellite observations of auroral electromagnetic fields and the formation of discrete auroral arcs.