FAST wave and particle observations on the nightside polar cap boundary indicate the operation of the ionospheric Alfven resonator (IAR). Large impulsive electric and magnetic field deviations on the boundary between the auroral oval and the polar cap close to magnetic midnight are correlated with accelerated electrons and excite semi periodic oscillations with a frequency of ~0.5 Hz. Linear one-dimensional simulations of the Alfven resonator including parallel electric fields due to electron inertial effects, the ionospheric feedback instability and statistically determined altitude dependent density and composition profiles in a dipole geomagnetic field yield waveforms and electron energy spectra qualitatively similar to observations. However, from comparison with a case study example observed above a sunlit ionosphere, the observed electron energies (which exceed 10 keV) suggest that the observed wave carries a parallel electric field larger than possible from electron inertial effects in the linear approximation particularly if this acceleration occurs at altitudes within the ionospheric Alfven resonator.