On intense diverging electric fields associated with black aurora

Abstract

Results are presented from the double‐probe electric field instrument on the Freja satellite with particular focus on the fine‐structured and dynamic plasma of the upper auroral ionosphere. The high‐resolution measurements show frequently occuring intense and irregular fine‐scale electric fields similar to those observed at higher altitudes by, for example, the S3‐3 and Viking satellites. Whereas the high‐altitude fields tend to be directly related to the auroral fine‐structure this is not always the case for the low‐altitude fields as illustrated by high‐resolution data of a pair of very intense (≈ 1 V/m), narrow electric field structures in the post‐midnight sector in the large‐scale downward field‐aligned current region. The structures are found to be associated with an excess of positive space charge (diverging electric fields), dropouts of precipitating electrons as well as depletions of thermal plasma, and significant wave activity. Combined with the scale‐size of the structures (≈ 1 km) and the spacing between them (≈ 5 km) these observations suggest that the intense electric fields are related to east‐west aligned vortex street structures of black aurora, similar to auroral curls but with opposite sense of rotation (clockwise seen antiparallel to B) and a total absence of auroral emissions. The detection of these structures was made possible by the relatively low inclination of the Freja orbit, which at times is almost tangential to the auroral oval. Thus, the Freja orbit provides a new perspective for studying many nightside auroral phenomena both at smaller scales (vortices) and at larger scales as exemplified by observations of north‐south oriented auroral structures caused by rotational arc distortions.