ON LOW-ALTITUDE PARTICLE-ACCELERATION AND INTENSE ELECTRIC-FIELDS ANDTHEIR RELATIONSHIP TO BLACK AURORA

Recent findings by the Freja satellite have shown the existence of ext remely intense (1-2 V/m) and small-scale (1 km) diverging electric fie lds which are interpreted to be associated with east-west aligned dark striations or black auroral curls. Precipitating or transversely ener gized ions, downward field-aligned currents carried by upward fluxes o f ionospheric electrons and dropouts of energetic electron precipitati on, are found to be characteristic features of such events. A comparis on of these characteristics to those of the aurora point at a symmetry between the aurora and the black aurora, the aurora being associated with negative divergence of the electric field and the black aurora wi th positive divergence. The diverging field events typically occur dur ing winter conditions within the midnight to early morning sector of t he auroral oval. Estimates of the ambient conductivity due to solar EU V radiation for each of these events show a clear anticorrelation with the electric field magnitude. The black auroral structures are likely to be associated with localized ionospheric density depletions below that of the ambient density and caused by the upward flow of ionospher ic electrons. The efficiency by which such density holes are created i n regions of downward field-aligned current flow have recently been de monstrated in model studies. The electric field magnitudes are found t o decrease with the scale size, not inversely as suggested in recent t heoretical work but with a power law exponent of 0.6-0.8. At lower alt itudes (around 800 km) the maximum intensities for a majority of the e vents are in the range of values that have been reported from rocket a nd radar measurements in the ionosphere, i.e., around 150-200 mV/m. Ho wever, close to magnetic midnight and during winter conditions small-s cale diverging electric fields of 1 V/m are occasionally found to exis t down to at least 800 km. We suggest that the diverging electric fiel ds observed by Freja are associated with low-altitude and narrow ( app roximate to 1-2 km) potential structures similar to the auroral potent ial structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by F reja observations of narrow upward beams of 2 keV electrons in good ag reement with a 2 kV positive peak in the electrostatic potential for a black aurora event. The existence of a downward parallel electric fie ld at low altitudes is also supported by low-altitude observations by the S3-3 and Viking satellites. If such low-altitude potential structu res do exist as our results suggest, an outstanding problem for future investigation is how they may be formed and maintained.