Observations of small‐scale auroral vortices by the S3‐2 satellite

Abstract

We have studied two intense auroral events which were encountered by the S3‐2 satellite at ∼0550 and ∼1930 MLT, during a substorm, near the equatorward edge of the region 1 current system. The events are marked by large deflections in the east‐west magnetic field component. In the dawn (dusk) event the deflection was 100 (300) nT with a total duration of 4(7) s. In both cases the deflection corresponding to an upward current sheet was sharpest, occuring over 0.25 s. Upward current strengths of 45 and 135 μA/m² within latitude extents of <2 km are inferred. The principal region of return current is not contiguous with the upward current sheet and is more spatially extended, with maximum intensities in the range of 10–15 μA/m². The events are associated with electric fields whose magnitude can exceed 200 mV/m. In the region between the current sheets, adjacent to the principal upward current sheet, the electric field rotated by 180° then returned to its original orientation. Such electric field variations give rise to plasma vortex flow patterns similar to these observed in auroral folds and curls. Despite spatial aliasing, the measurements of an electron detector in the vicinity of the upward current sheets provide useful information concerning the density and temperature of the parent populations and the field‐aligned potential drop. With some reasonable assumptions, it is shown that observed values ofj∥ and Φ∥ are consistent with collisionless, single‐particle theories.