MODEL STUDY OF MULTIPLE POLAR-CAP ARCS - OCCURRENCE AND SPACING

A new scenario for the formation of multiple polar cap arcs is propose d based on the results from a time-dependent electrodynamic model of p olar cap arcs developed by Zhu et al. [1993]. The results suggest that the appearance of multiple polar cap arcs may not be due to multiple structures in the magnetospheric source region, but instead, may prima rily be determined by the coupled magnetosphere-ionosphere system in w hich the ionosphere plays an active role. It was found that with the s ame magnetospheric driver, a strong ionospheric background convection and an ionospheric background Hall conductance in the range of from 0. 5 to 1.5 mho favor the occurrence of multiple polar cap arcs. It was a lso found that the spacing between individual arcs is highly correlate d with the ratio of the enhanced Hall and Pedersen conductances (i.e., the hardness of the primary magnetospheric precipitation), which is a llowed to vary in the model runs. A harder precipitation causes a wide r spacing. In the modelings, the spacing ranged from about 20 kilomete rs to about 60 kilometers when the precipitation hardness increased.