SIMULTANEOUS GROUND AND ROCKET BASED MEASUREMENTS OF ELECTRIC FIELDS AND CURRENTS IN AN AURORAL ARC

A Terrier-Malemute sounding rocket was launched into a stable, premidnight auroral arc. Among the instruments carried by the payload were a cesium vapor magnetometer, electrostatic curved-plate electron energy analyzers, and two sets of Langmuir double-probes. Simultaneous line-of-sight velocity and electron density measurements were made during the flight by Chatanika radar operating in an elevation scan mode in the magnetic meridian. Data from the magnetometer indicated that a broad region of eastward current flowed within the diffuse aurora equatorward of the arc, while westward current flowed within the arc itself. The field aligned current was downward in the diffuse aurora and uniformly upward in a 100 km wide region which contained the arc. This current pattern suggests that the payload traversed the entire Region 1 field aligned current system. The narrowness of the current was probably due to the expansion of the auroral oval associated with the growth phase of a substorm which occurred one hour after the flight. The double probe measured a 40 mV/m northward electric field just equatorward of the arc. Within the arc the northward electric field was zero. The tangential component of the electric field was 7 mV/m and was constant across the arc boundaries. Since the electric field in the arc was not strong enough to account for the westward current measured by the magnetometer, the presence of a westward neutral wind was inferred. This westward wind is consistent with the difference between the meridional electric field measured by the double probe and the radar. Differences in the east-west electric field measured by the two instruments could be accounted for by an upward drift of ions at 100 m/s within the arc. This ion current constituted half of the measured upward field aligned current within the arc, while the other half was carried by precipitating electrons. The simultaneous measurements enabled the association of the aurora with large scale regions in the magnetosphere. They also allowed the construction of a self consistent perpendicular current system for the arc involving a time varying neutral wind. There was some discrepancy in the field aligned currents derived from the data. This was attributed to the arc's association with the Harang Discontinuity