In November 1985 an experimental campaign was conducted at Sondre Stromfjord, Greenland, to study the electrodynamics of auroral arcs at very high latitudes. The experiments involved coordinated observations by the Sondrestrom radar and two polar orbiting spacecraft. For several hours around local dawn on November 17 a system of auroral arcs was observed over Sondrestrom. These arcs varied in intensity and width but were all extended generally in the magnetic east-west direction. A Defense Meteorological Satellite Program photograph obtained during the observations showed that two types of arcs were present. In the south the arcs were localized enhancements or striations within a broad diffuse aurora. Poleward of the diffuse aurora was a system of arc segments that extended over about 5¿ of latitude. The radar measured enhanced ionospheric electron densities associated with each of these arcs, although the altitudes of the enhancements were different for the two types. In the diffuse aurora the ionization peaked at altitudes near 115 km, whereas in the arc segments the ionization peaked at altitudes above 180 km.

Examination of electric field data from the radar indicated that the boundary in ionization associated with the two types of aurora occurred where the southward electric field maximized. The HILAT satellite passed in the vicinity of Sondrestrom during the experiment and measured precipitating electron fluxes and field-aligned currents. The electron flux data showed that the low-altitude ionization observed by the radar was produced by electrons with energies near 5 keV. The higher-altitude ionization was produced by precipitation associated with enhancements in the fluxes of low-energy electrons. The magnetometer data indicated that the boundary between the region 1 (downward) and region 2 (upward) current sheets in the morning sector is just poleward of the boundary separating the hard and soft precipitation regions. The latitudinal variation in field-aligned current is consistent with the measured variations in electric field and conductivity. ¿ American Geophysical Union 1988