Dispersive effects in field line resonances (FLRs) are discussed in the context of potential structures, parallel currents, and auroral density cavities observed by the FAST satellite. Our model includes the Earth's dipole magnetic field, and accounts for electron inertia, electron thermal pressure, finite ion gyroradius effects, and field aligned variations of the plasma density and ambient electron and ion temperatures. For realistic background parameters, we show that finite plasma temperature effects determine the dynamics of FLRs and that solitary wave structures evolve out of the resonance region, producing deep density cavities above the polar ionospheres. Results are shown to be in reasonable agreement with ground and satellite observations, with the exception of the magnitude of low altitude electric fields. ¿ 1999 American Geophysical Union