Suppression of convective cell generation by Alfvén waves in the ionospheric auroral cavity

Abstract

A theory for the parametric interaction of inertial Alfvén wave (IAW) packets with large-scale convective cells accounting for the nonzero ratio of the Alfvén and light velocities is presented. It is shown that in the auroral plasma cavity where the Alfvén velocity has its maximum value, the parametric interaction of IAWs with convective cells is substantially reduced. In this region the Alfvén velocity is comparable to the speed of light and thus the displacement current cannot be neglected. This current plays a stabilizing role and results in a decline of the parametric instability growth rate. On the contrary, the incorporation of the finite width of the IAW packet leads to increase in the convective cell growth rate. A compact expression for the instability growth rate and the optimal conditions for the fastest growing convective mode are derived. The relevance of the theory to recent spacecraft measurements in the Earth’s ionosphere is pointed out.