THE AURORAL CURRENT CIRCUIT AND FIELD-ALIGNED CURRENTS OBSERVED BY FAST

FAST observes signatures of small-scale downward-going current at the edges of the inverted-V regions where the primary (auroral) electrons are found. In the winter pre-midnight auroral zone these downward curr ents are carried by upward flowing low- and medium-energy (up to sever al keV) electron beams. FAST instrumentation shows agreement between t he current densities inferred from both the electron distributions and gradients in the magnetic field. FAST data taken near apogee (similar to 4000-km altitude) commonly show downward current magnetic field de flections consistent with the observed upward flux of similar to 10(9) electrons cm(-2) s(-1), or current densities of several mu A m(-2). T he electron, field-aligned current and electric field signatures indic ate the downward currents may be associated with ''black aurora'' and auroral ionospheric cavities. The field-aligned voltage-current relati onship in the downward current region is nonlinear.