Akebono (Exos D) observations demonstrate that polar cap arcs sometimes have a fine structure, that is, multiple (double or triple) arcs with spacing of a few tens of kilometers. The multiple polar cap arcs are dominantly observed in the nightside polar cap region, suggesting that low background conductance favors the appearance of the structured arcs. A relationship between the spacing and the average energy of the precipitating electrons is investigated. Results show that a higher energy leads to a wider spacing. Akebono observations also show the existence of a downward current region embedded between upward current regions (arcs). Comparison of the observations with results from a coupled magnetosphere-ionosphere Sun-aligned arc model is made, which shows good qualitative agreement between the modeling and observational results on the spacing-energy dependence and the effect of background ionospheric conductance.
Obara, T., T. Mukai, H. Hayakawa, K. Tsuruda, A. Matsuoka, A. Nishida, H. Fukunishi, L. Zhu, J. Sojka, and D. Crain (1996), Multiple polar cap arcs: Akebono (Exos D) observations, Radio Sci., 31(3), 645–653, doi:10.1029/96RS00435.