Electrodynamics in the Polar Ionosphere During the IMF-north Substorm on 13 December 2004

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 electrojet 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 electrojet.