The structure of auroral arcs sustained by field line resonances (FLRs) is determined using a model that describes the interplay between ionospheric feedback, nonlinear, and dispersive effects in a curvilinear geomagnetic topology. The model includes modulation of Pedersen conductance by hundreds of eV electrons that precipitate in the ionosphere through the action of shear Alfv¿n wave field-aligned currents (FACs). The competition between ionospheric feedback dissipation, wave dispersion, and nonlinearity results in large-amplitude, long-period oscillations of the FAC, and in emission of slow-moving small-scale secondary auroral arcs and density perturbations. Using observed values of nightside conductivities and realistic topology of geomagnetic field lines, we obtain FLRs with frequencies in the range of a few mHz, spatial scales up to several km near the ionosphere, and FAC amplitudes extending to tens of ¿A/m2. Our model explains the excitation of structured auroral arcs in regions of low ionospheric conductance.