PROGRESS ON RELATING OPTICAL AURORAL FORMS AND ELECTRIC-FIELD PATTERNS

Ultraviolet auroral; imagery and in situ electric field measurements f rom Freja are presented and used to identify signatures of the perpend icular electric field associated with three basic are configurations. The auroral imagery is essential in identifying the two-dimensional ch aracteristics of the discrete auroral forms for which electric field s ignatures 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 are or as a part of a series of parall el arcs within the dusk and premidnight auroral distribution. The scal e size of the arcs is given by their thickness, typically similar to 1 00 km. Common electric field signatures, such as a reduction in the co nvection electric field in the region of the are and enhancements of t he convection electric field at the boundaries of the are, are observe d. The electric field reductions dominating the signatures of quiet ar cs occurring as a single are indicate that these arcs are characterize d with a weak held-aligned current. As inferred from the electric fiel d 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 d isturbed arcs are the quasi-periodic fluctuations in the disturbed arc 's electric field signature. The turbulence in the electric field indi cates that shear structures and shear-driven plasma instabilities are involved in the formation of auroral spirals.