It is shown that the negative ohmic dissipation associated with the ion drift wave in the ionosphere under the presence of the large-scale stationary electric field results in the shear Alfv¿n instability in the magnetosphere. A criterion of the instability is derived from a simple energetic consideration, which requires the reverse sign of the perturbed electric field in the ionosphere comparing with that at the equator. In fact, numerical solutions of the eigen-mode oscillations in realistic ionosphere-magnetosphere coupling system indicate such a change of sign between the two and also yield an upward field-aligned Poynting flux from the ionosphere for the unstable oscillations, while for the damping oscillations we have the downward energy flow along a field line.