We have examined characteristics of the field-aligned currents in the poleward boundary region of the nightside auroral oval, using magnetic field, electric field, and particle data obtained from the Akebono satellite. We found that large-amplitude, wave-like fluctuations of magnetic and electric fields frequently occur at mid-altitudes confined in this latitudinally narrow region. We examined the relationship between electric and magnetic field fluctuations, examined using the complex impedance function defined as Z˜=&mgr;0E˜x/B˜y, where E˜x and B˜y are the X component of the electric field and the Y component of the magnetic field, respectively. The most important result obtained from this analysis is that the features of the impedance functions in the frequency range between 2¿10-3 and 8¿10-2 Hz are well explained by a time-dependent magnetosphere--ionosphere coupling model. Further, the calculation of the field-aligned Poynting flux showed that most of the Alfven wave energy is reflected at the ionosphere rather than dissipated in the ionosphere. These facts suggest that the majority of the field-aligned current fluctuations observed at mid-altitudes in the poleward boundary region of the nightside auroral oval are due to the superposition of incident and reflected Alfven waves. It is speculated that the breaking of reflected Alfven waves in the PSBL may cause the heating of plasma sheet electrons and ions. ¿ American Geophysical Union 1996