ARIA II neutral flywheel-driven field-aligned currents in the postmidnight sector of the auroral oval: A case study

The ARIA II experiment carried out on February 12, 1994, provided the neutral wind and electron density measurements needed to calculate the neutral‐driven field‐aligned current densities. In situ rocket measurements were made approximately 60 min after the onset of an auroral substorm in a moderately disturbed (Kp=5−) postmidnight auroral oval over Alaska. Two chemical release rockets deployed four widely separated TMA trails, and E region neutral wind profiles were obtained from the motion of those trails. An instrumented rocket launched near‐simultaneously measured the electric fields, electron densities, and neutral composition. The divergence and the vertical component of the vorticity in the neutral flow were obtained from the neutral wind profiles, and those values, together with the measured electron densities, were used to calculate the neutral‐driven field‐aligned current densities. Our best estimate is that the current density below 114 km was effectively zero. Above 114 km the field‐aligned current was downward with a peak value 0.23 μA m−2. Our results indicate that, although the wind‐driven current was a significant contributor to the total field‐aligned current in the recovery phase of the substorm, the dominant sources of the field‐aligned current were due to conductivity gradients and the divergence in the ionospheric electric field. The measurements also show that the current density profile has vertical structure with a scale of ∼ 10 km due to height variations in the vorticity, divergence, and conductivity profiles.