Progress on relating optical auroral forms and electric field patterns

Abstract

Ultraviolet auroral imagery and in situ electric field measurements from Freja are presented and used to identify signatures of the perpendicular electric field associated with three basic arc configurations. The auroral imagery is essential in identifying the two‐dimensional characteristics of the discrete auroral forms for which electric field signatures are examined. The high‐resolution data show the differences between the electric field signatures of the quiet and disturbed arcs (spirals) occurring as a single arc or as a part of a series of parallel arcs within the dusk and premidnight auroral distribution. The scale size of the arcs is given by their thickness, typically ∼ 100 km. Common electric field signatures, such as a reduction in the convection electric field in the region of the arc and enhancements of the convection electric field at the boundaries of the arc, are observed. The electric field reductions dominating the signatures of quiet arcs occurring as a single arc indicate that these arcs are characterized with a weak field‐aligned current. As inferred from the electric field enhancements dominating the signature of quiet arcs which are a part of a series of arcs, these arcs are characterized with a strong field‐aligned current. Features which distinguish the quiet arcs from the disturbed arcs are the quasi‐periodic fluctuations in the disturbed arc's electric field signature. The turbulence in the electric field indicates that shear structures and shear‐driven plasma instabilities are involved in the formation of auroral spirals.