Direct observation of large, quasi-static, parallel electric fields in the auroral acceleration region

The three-axis electric field experiment on the Polar satellite provides direct observations of electric field components parallel and perpendicular to the local magnetic field with no arbitrary adjustment parameters. Approximately 750 perigee passes through each of the two southern auroral zones at a geocentric altitude of about two Earth radii have been computer-searched for parallel electric fields whose eight point (0.2 or 0.4 second) average exceeded 100 mV/m. After elimination of spurious events due to shadowing, saturation, and ten other effects, four events containing parallel fields of 200-300 mV/m, remain. These four events all occur in upward field aligned current regions, their parallel electric fields are all positive such that (j) over right arrow.(E) over right arrow > 0, and they occur at boundaries between regions of active and quiet perpendicular electric fields. Up-going ion beams are observed in the active field regions, and the plasma density is higher in the quiet field regions than in the adjacent active field regions. These boundaries are interpreted in terms of model equipotentials, some of which are below the spacecraft in the large field regions and all of which are above the spacecraft in the quiet field regions. In this model, the expected location of large parallel electric fields is where they are observed. That the potential difference measured by the electric field instrument along the vehicle trajectory and the kinetic energy of the up-going ions are equal lends further credence to the data and its interpretation.