FIELD-ALIGNED CURRENTS ASSOCIATED WITH ALFVEN WAVES IN THE POLEWARD BOUNDARY REGION OF THE NIGHTSIDE AURORAL OVAL

We have examined characteristics of the field-aligned currents in the poleward boundary region of the nightside amoral oval, using magnetic field, electric field, and particle data obtained from the Akebono sat ellite. We found that large-amplitude, wave-like fluctuations of magne tic 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) over tilde = mu(o) (E) over tilde(x) /(B) over tilde(y), where (E) over tilde(x) and (B) over tilde(y) are the X component of the electric field and the Y component of the magne tic field, respectively. The most important result obtained from this analysis is that the features of the impedance functions in the freque ncy range between 2 x 10(-3) and 8 x 10(-2) Hz are well explained by a time-dependent magnetosphere-ionosphere coupling model. Further, the calculation of the held-aligned Poynting flux showed that most of the Alfven wave energy is reflected at the ionosphere rather than dissipat ed in the ionosphere. These facts suggest that the majority of the fie ld-aligned current fluctuations observed at mid-altitudes in the polew ard boundary region of the nightside amoral oval are due to the superp osition of incident and reflected Alfven waves. It is speculated that the breaking of reflected Alfven waves in the PSBL may cause the heati ng of plasma sheet electrons and ions.