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 measur ements were made approximately 60 min after the onset of an auroral su bstorm in a moderately disturbed (Kp=5(-)) postmidnight auroral oval o ver Alaska. Two chemical release rockets deployed four widely separate d TMA trails, and E region neutral wind profiles were obtained from th e motion of those trails. An instrumented rocket launched near-simulta neously measured the electric fields, electron densities, and neutral composition. The divergence and the vertical component of the vorticit y in the neutral flow were obtained from the neutral wind profiles, an d those values, together with the measured electron densities, were us ed to calculate the neutral-driven field-aligned current densities. Ou r best estimate is that the current density below 114 km was effective ly zero. Above 114 km the field-aligned current was downward with a pe ak value 0.23 mu A m(-2). Our results indicate that, although the wind -driven current was a significant contributor to the total field-align ed current in the recovery phase of the substorm, the dominant sources of the field-aligned current were due to conductivity gradients and t he divergence in the ionospheric electric field. The measurements also show that the current density profile has vertical structure with a s cale of similar to 10 km due to height variations in the vorticity, di vergence, and conductivity profiles.