Coupling of the magnetopause to the ionosphere by means of alfvén waves and field-aligned currents

Remote sensing of magnetopause processes by the observation of their signatures in the ionosphere requires an understanding of the coupling between these two regions. A major magnetopause- ionosphere coupling mechanism is the generation and propagation of field-aligned currents. Optical and particle signatures are also influenced by the current structures, since current-carrying flux tubes can lead to acceleration of particles into the ionosphere. This paper will focus on the mechanisms for the generation of field-aligned current at the magnetopause and their consequences at the ionosphere. Current generation by solar wind dynamic pressure pulses, the Kelvin- Helmholtz instability, and magnetic reconnection will be considered in detail. Pressure pulses can lead to the production of large scale convection vortices in the ionosphere, with properties similar to those obtained by numerical simulation. On the other hand, Kelvin-Helmholtz instabilities can be stabilized by the ionospheric conductivity as well as by magnetic forces associated with field line curvature. In addition, the Kelvin-Helmholtz mechanism cannot account for the sense of winding of auroral spirals, and so models which invoke the Kelvin-Helmholtz instability at the dayside magnetopause should be considered carefully. Localized magnetic reconnection, which can be produced by impulsive processes at the magnetopause, can give rise to field-aligned currents which depend on the B_Y component of the interplanetary magnetic field, and is the likely cause of the cusp current system.