VORTEX MOTION IN THE IONOSPHERE AND NONLINEAR TRANSPORT

The relation between vorticity and ionospheric flow patterns is investigated by using a fluid mechanics approach in place of the more customary electromagnetic approach. The focus on the fluid features is justified by the observation that in the incompressible limit appropriate to the ionosphere, vorticity can be regarded as the source of the flow field. We show how vorticity can be introduced into the flow by local ionospheric conditions. However, in the cases of greatest interest, the vorticity is imposed by external sources, which can be in the magnetosphere or in the solar wind. As an important application, we consider traveling ionospheric vortices propagating around the polar cap boundary. We show that such traveling disturbances transport both momentum and magnetic flux in the direction of their phase velocity, typically antisunward. Like other intermittent disturbances of small scale, such as flux transfer events, individual traveling ionospheric vortices transport relatively little flux, but multiple disturbances could conceivably transport an important fraction of the polar cap magnetic flux from the dayside to the tail.