The role of field-aligned currents patterns in connection with magnetospheric structure

Recent studies of digitally sampled two-axis search coil magnetometer data from Siple Station, Antarctica have revealed an unusual class of asymmetric pulsations at the equatorward edge of the morning sector auroral zone. We have reviewed three-axis data taken with this same instrumentation in 1974 to further characterize the vector properties of these one-sided pulsations and to place further constraints on possible source mechanisms. Data will be presented showing that ionospheric and filamentary field-aligned currents, as modeled by Arnoldy et al. [1982] and Engebretson et al. [1983a] are likely to be major contributors to these Pi 1 signatures. If these models are correct, filamentary structures in morning sector Region II Birkeland currents are produced by the ionospheric response to localized impulsive precipitation of energetic electrons from trapped orbits in the magnetospheric plasma sheet (pulsating aurora).