AURORAL PHENOMENA RELATED TO INTENSE ELECTRIC-FIELDS OBSERVED BY THE FREJA SATELLITE

Electrodynamical features of the black aurora, and its optical counter part, the aurora, are discussed on the basis of electric held, magneti c field and particle observations by Freja. Extraordinarily intense (1 -2 V m(-1)) and small-scale (1-5 km) electric fields have been observe d by the Freja satellite mostly in association with black aurora (vort ices and east-west dark filaments) but also in association with aurora l structures such as spirals and surges. The former structures are cha racterized by diverging electric fields, dropouts of energetic electro n precipitation, precipitating or transversely energized ions and down ward field-aligned currents carried by upward fluxes of ionospheric el ectrons, opposite to the characteristics of the aurora. The black auro ral structures are associated with localized ionospheric density deple tions below that of the ambient density and the field magnitude is fou nd to be anticorrelated to the conductivity. During conditions of mini mum conductivity small-scale diverging electric fields of 1 V m(-1) oc casionally exist down to 800 km. We suggest that the diverging electri c fields observed by Freja are associated with low-altitude and narrow (approximate to 1-2 km) potential structures similar to the auroral p otential structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by Freja observations of intense upward electron beams and positive p otential peaks as well as by low-altitude electric field observations by the S3-3 and Viking satellites. The electric field characteristics of auroral surges and large-scale auroral spirals are very different f rom those of the black aurora. This will be exemplified by high-resolu tion electric field, magnetic held, particle and UV-imager observation s from oblique Freja crossings of the eveningside auroral oval. The am bient electric field is found to intensify in the direction towards th e centre of the auroral spiral which confirm, previous findings that t he surge and spiral head is associated with negative space charge and an intense upward field-aligned current. A pass directly through the s urge head, however, reveals a very inhomogeneous and complicated pictu re of the surge such as narrowly structured, intense (up to 700 mV m(- 10) ) converging electric fields, intense electron precipitation and b alanced field-aligned currents (up to 30 mu A m(-2)) embedded within a n extended region of intense high-energy electron precipitation, weak electric fields and held-aligned currents. According to some surge mod els, a pronounced westward electric held component and a southward pol arization electric field is expected within the entire high-conductivi ty region but evidence in support of this was not found in the data. R ather, these suggest that a significant part of the upward surge curre nt is closed by distributed downward held-aligned currents from the ne ar surroundings. The surge electric field is much more intense than pr eviously observed or anticipated at these altitudes, having characteri stics rather similar to those observed in the auroral acceleration reg ion.