MULTIPLE POLAR-CAP ARCS - AKEBONO (EXOS-D) OBSERVATIONS

Akebono (Exos D) observations demonstrate that polar cap arcs sometime s have a fine structure, that is, multiple (double or triple) arcs wit h spacing of a few lens of kilometers. The multiple polar cap arcs are dominantly observed in the nightside polar cap region, suggesting tha t low background conductance favors the appearance of the structured a rcs. A relationship between the spacing and the average energy of the precipitating electrons is investigated. Results show that a higher en ergy leads to a wider spacing. Akebono observations also show the exis tence of a downward current region embedded between upward current reg ions (arcs). Comparison of the observations with results from a couple d magnetosphere-ionosphere Sun-aligned are model is made, which shows good qualitative agreement between the modeling and observational resu lts on the spacing-energy dependence and the effect of background iono spheric conductance.