  High resolution optical measurements have revealed that a sudden brightening
of aurora and its deformation from an arc-like to a vortex street structure
appear just at the onset of substorm. The instability of Alfv$\acute{\rm e}$n
waves reflected from the ionosphere has been studied by means of
magnetohydrodynamic simulations in order to comprehend the formation of auroral
vortex streets. Our previous work reported that an initially placed arc
intensifies, splits, and deforms into a vortex street during a couple of
minutes, and the prime key is an enhancement of the convection electric field.
This study elaborated physics of the ionospheric horizontal currents related to
the vortex street in the context of so-called Cowling polarization. One
component 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 the
ionosphere. It was found that, when a vortex street develops, upward/downward
pair currents in its leading/trailing sides balance with an westward polarized
component of the Hall current; it generates an eastward perturbed component of
the Pedersen current. It was also found that both the perturbed component of
the Hall current and the polarized component of the Pedersen current point
equatorward, penetrating between the pair currents.
