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High resolution optical measurements have revealed that a sudden brighteningof aurora and its deformation from an arc-like to a vortex street structureappear just at the onset of substorm. The instability of Alfv$\acute{\rm e}$nwaves reflected from the ionosphere has been studied by means ofmagnetohydrodynamic simulations in order to comprehend the formation of auroralvortex streets. Our previous work reported that an initially placed arcintensifies, splits, and deforms into a vortex street during a couple ofminutes, and the prime key is an enhancement of the convection electric field.This study elaborated physics of the ionospheric horizontal currents related tothe vortex street in the context of so-called Cowling polarization. Onecomponent is due to the perturbed electric field by Alfv$\acute{\rm e}$n waves,and the other is due to the perturbed electron density (or polarization) in theionosphere. It was found that, when a vortex street develops, upward/downwardpair currents in its leading/trailing sides balance with an westward polarizedcomponent of the Hall current; it generates an eastward perturbed component ofthe Pedersen current. It was also found that both the perturbed component ofthe Hall current and the polarized component of the Pedersen current pointequatorward, penetrating between the pair currents.

Ionospheric current system accompanied by auroral vortex streets