The stabilizing effect of collision-induced velocity shear on the ionospheric feedback instability in Earth's magnetosphere

The feedback instability in the ionospheric Alfven resonator in Earth's magnetosphere is examined using a two-dimensional multifluid numerical model of coupled ionosphere and magnetosphere. Two simulation configurations are used to demonstrate that the instability occurs under an assumption that is unrealistic for Earth's ionosphere. In the first configuration, a flat sheet height-integrated conducting boundary replaces the ionospheric E layer. In the second configuration, plasma dynamics in a simplified E layer is resolved ignoring ion production, loss, and diffusion. For the same parameters (plasma and neutral density profiles and convection electric field), the instability develops only with the flat sheet boundary. When the E layer is resolved, the variation of ion-neutral collision frequencies with altitude produces vertical shear in the horizontal ion flow velocity. The shear prevents density perturbations from remaining field aligned, causing them to decay rather than grow. It is suggested that the instability cannot occur in Earth's ionosphere because ion-neutral collision frequencies always have a significant variation with altitude through the E layer.