Observations by the instrument payload on DE-2 have been used to study the plasma and electrodynamic properties of the ionosphere when field-aligned currents approaching 100 &mgr;A m-2 flow at about 900-km altitude. In such a situation it is found that the thermal ions may account for a substantial fraction of the current carriers and that the bulk of the energetic electron population has undergone a net acceleration of a few hundred electron volts. Large amplitude electrostatic waves that have a maximum near 100 Hz and an energetic ion population that shows evidence for both parallel and perpendicular acceleration to about 30 eV also accompany such events. The horizontal thermal ion drift (electric field) that accompanies these plasma phenomena has extremely large gradients over small spatial scales. Such a signature is consistent with a requirement for a large field-aligned current and a limit to the available field-aligned difference that may be applied to the plasma.