Previous studies have shown that substorm can occur even during northward Interplanetary Magnetic Field （IMF）. However, the electrodynamics in the polar ionosphere has not been studied for this kind of events. In this paper, the KRM magnetogram-inversion algorithm, combined with the magnetic record at the north hemisphere, was utilized to deduce the distribution of the current vector, the current function, and the electric potential in the polar ionosphere during the substorm event on 13 Dec. 2004. The results show a sudden enhancement of the westward electro jet at the midnight sector after the expansion onset. The equivalent current system was characterized as a two-vortex structure at the nightside, in association with a southward electric field. All these indicate an absolutely dominant role of the unloading process at the expansion phase. The two-cell convection structure could not be found, which suggests a weak directly driven process in association with the northward IMF. The contribution of the conductivity and electric field to the enhancement of the current were also examined. The results showed that the enhancement of the conductivity is responsible for the increase of the westward electro jet.%利用KRM地磁反演方法，结合北半球中高纬度地磁台站数据，研究了2004年12月13日行星际磁场北向期间发生的亚暴事件，极区电离层电动力学参量（电流矢量、等效电流函数以及电势）的分布特征．结果表明，在该亚暴膨胀相起始后，午夜之前西向电集流急剧增强，且等效电流体系表现为夜侧双涡，同时伴随夜侧增强的南向电场．由于极弱的直接驱动过程，卸载过程引起的电离层效应得到清楚显示．卸载过程在膨胀相期间起绝对主导性作用．同时，夜侧电导率的增强是电集流区域电流急剧增强的主要原因．
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