Electrodynamic structure of the late evening sector of the auroral zone

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Abstract
Detailed observations from a late evening sector pass of S3‐2 over the northern auroral zone are presented and compared with nearly simultaneous DMSP imagery. High time resolution measurements are presented from electric field, magnetic field, energetic electron (80 eV ≤E ≤17 keV), and thermal plasma detectors. The region of diffuse aurorae was characterized by nearly isotropic electron fluxes and slowly varying (≤20 mV/m) electric fields. The Birkeland currents in this region were predominantly into the ionosphere with embedded, narrow sheets of upward currents. Simultaneous magnetometer and satellite potential fluctuations show that the upward current sheets are real and that these upward currents are probably carried by precipitating, low‐energy plasma sheet electrons. Poleward of the diffuse aurorae, the covective electric field varied rapidly in magnitude, anticorrelating with the intensity of precipitating electrons. An inverted‐V structure was observed above a visible arc. An intense downward current sheet (>10 µA/m²) was found near the equatorward edge of the arc. At this place the ionospheric thermal plasma was near marginal stability for the onset of O+ ion cyclotron turbulence. An electric field component parallel to B of ∼10 mV/m was measured in this region of strong downward current. The plasma and electric field distributions in the vicinity of the arc suggest that the auroral electrojet was distributed over the region of diffuse auroral but was most intense near the equatorward boundary of the auroral arc.