The auroral current circuit and field‐aligned currents observed by FAST

Abstract

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 currents are carried by upward flowing low‐ and medium‐energy (up to several keV) electron beams. FAST instrumentation shows agreement between the current densities inferred from both the electron distributions and gradients in the magnetic field. FAST data taken near apogee (∼4000‐km altitude) commonly show downward current magnetic field deflections consistent with the observed upward flux of ∼10 9 electrons cm −2 s −1 , or current densities of several µA m −2 . The electron, field‐aligned current and electric field signatures indicate the downward currents may be associated with “black aurora” and auroral ionospheric cavities. The field‐aligned voltage‐current relationship in the downward current region is nonlinear.