MODEL-OBSERVATION COMPARISON STUDY OF MULTIPLE POLAR-CAP ARCS

A quantitative model-observation comparison of multiple polar cap arcs has been conducted by using a time-dependent theoretical model of pol ar cap arcs. In particular, the electrodynamical features of multiple polar cap arcs with various spacings are simulated and the results are compared with the images obtained from the All-Sky Intensified Photom eter at Qaanaaq. The results show that the observed and simulated arcs are quite similar, both spatially and temporally. The results support the theory proposed by Zhu et al. [1993a, l994b] that the structure o f polar cap arcs is mainly determined by the magnetosphere-ionosphere (M-I) coupling processes and that the spacing of multiple polar cap ar cs is closely related to the hardness of the primary magnetospheric pr ecipitation. It is found that for the multiple polar cap arcs with bot h narrow and wide spacings, the associated field-aligned currents are mainly closed by Pedersen currents. It is also found that a hard preci pitation can lead to a highly structured secondary are because of the nonlinear M-I coupling processes.