Temporal evolution of a two-dimensional electrostatic shear instability is numerically studied with special attention to rotational deformation of auroral arcs and momentum transport across the magnetospheric boundary. It is found that the spatial structure of the growing vortex is in good agreement with the small-scale auroral deformation called 'curl.' An anomalous viscosity arising from the shear instability is found to be given approximately by 0.08 aVo, where a is the characteristic distance over which the flow velocity changes by Vo. If we assume that a=200 km and Vo=250 km/s at the magnetospheric boundary, the anomalous viscosity becomes 4¿1013 cm2 s-1, which is comparable to, or even larger than, the value estimated by Axford (1964) from energy requirements of a typical magnetic storm. Spectral energies of fully developed electro-static turbulence are also studied. It is found that the observed power spectral energies can well be represented by a power law of the form k-4.