Auroral phenomena related to intense electric fields observed by the Freja satellite

Electrodynamical features of the black aurora, and its optical counterpart, the aurora, are discussed on the basis of electric field, magnetic field and particle observations by Freja. Extraordinarily intense (1-2 V m(-1)) and small-scale (1-5 km) electric fields have been observed by the Freja satellite mostly in association with black aurora (vortices and east-west dark filaments) but also in association with auroral structures such as spirals and surges. The former structures are characterized by diverging electric fields, dropouts of energetic electron precipitation, precipitating or transversely energized ions and downward field-aligned currents carried by upward fluxes of ionospheric electrons, opposite to the characteristics of the aurora. The black auroral structures are associated with localized ionospheric density depletions below that of the ambient density and the field magnitude is found to be anticorrelated to the conductivity. During conditions of minimum conductivity small-scale diverging electric fields of 1 V m(-1) occasionally exist down to 800 km. We suggest that the diverging electric fields observed by Freja are associated with low-altitude and narrow (approximate to 1-2 km) potential structures similar to the auroral potential 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 potential 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 from those of the black aurora. This will be exemplified by high-resolution electric field, magnetic field, particle and UV-imager observations from oblique Freja crossings of the eveningside auroral oval. The ambient electric field is found to intensify in the direction towards the centre of the auroral spiral which confirm, previous findings that the surge and spiral head is associated with negative space charge and an intense upward field-aligned current. A pass directly through the surge head, however, reveals a very inhomogeneous and complicated picture of the surge such as narrowly structured, intense (up to 700 mV m(-10)) converging electric fields, intense electron precipitation and balanced field-aligned currents (up to 30 mu A m(-2)) embedded within an extended region of intense high-energy electron precipitation, weak electric fields and field-aligned currents. According to some surge models, a pronounced westward electric field component and a southward polarization electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The surge electric field is much more intense than previously observed or anticipated at these altitudes, having characteristics rather similar to those observed in the auroral acceleration region.