Recently, much attention has focused on physical mechanisms responsible for the formation and evolution of the plasma density irregularities in the F region ionosphere that disrupt communications at high latitudes. The polar orbiting S3–2 satellite with its complement of high resolution of plasma, particle and field detectors provides useful information on the dynamics of some irregularities. This study focuses on plasma density and electric field measurements taken as the satellite flew poleward along the Harang discontinuity during a substorm. The position of the Harang discontinuity is established from simultaneous ground magnetometer measurements and DMSP imagery. The plasma density showed a distinct maximum near the poleward boundary of auroral electron precipitation that was two orders of magnitude higher than in the trough and in the (winter) polar cap. Very intense (>100 mV/m) electric field fluctuations were detected along the steep, equatorward, plasma density gradient near the poleward boundary of the oval. Power spectral densities of these fluctuations did not obey expected power laws but had strong spectral lines between 1 and 16 Hz that do not seem to map to the peak of the F-layer. This suggests a magnetospheric source for at least some high-latitude electric field irregularities. Simultaneously measured spectra for plasma densities consistently showed k−2 power laws over the 0- to 8-Hz range. This is consistent with the predictions of nonlinear, gradient drift theory.