A Black Brant VB sounding rocket was launched at 03h 56m 05s UT on August 17, 1970, into the expansion phase of a major magnetospheric substorm during which the perturbation magnetic field was in excess of 2000 &ggr; at ground level. Measurements of the electric field component perpendicular to the earth's magnetic field made by double-probe detectors on board the rocket and by observation of the motion of two barium ion clouds released from the same rocket were in excellent agreement. The rocket remained within a region of intense particle precipitation where the field was 15¿--45¿ south of geomagnetic west with a magnitude between 60 mV/m and 90 mV/m. The barium clouds drifted southeast into a region of weak aurora and smaller electric field. These measurements show that the electric field is not always smallest inside auroral arcs and that the poleward auroral expansion phase is not accompanied by poleward ionospheric plasma motion and suggest a divergence of the northward perpendicular ionospheric current and hence that a downward field-aligned current was present in the equatorward portion of the arc structure. During a portion of the flight the double-probe detectors also measured a parallel component of electric field between 10 and 24 mV/m which pointed downward toward the earth. The total electric field detected on board the rocket was deduced by two techniques: one technique which does not involve assumptions involving dc offsets of the detector but relies on the precessional motion of the rocket to separate E⊥ and E∥ and one high time resolution technique which involves an assumption that these offsets approach a constant value in the flight. The same basic conclusions were deduced from both techniques. When the possibility of current-driven anomalous resistivity due to ion acoustic waves is taken into account, the measurements are in agreement with a slow parallel motion of ambient plasma ions (<10 m/s). However, when the theory is applied to a two-component plasma, the heavier, lower-density species should be accelerated to speeds much in excess of the observed values for the barium ions. Thus the low parallel barium ion velocity which was observed is not in agreement with an ion acoustic wave explanation for the observed parallel electric field. Although the evidence is strong that a parallel component of electric field was detected, one cannot rule out the possibility that this field-aligned potential difference was created by the motion of the vehicle through the medium.