The merging of the solar wind with the Earth's magnetic field is now accepted as being the principal mechanism by which solar wind energy is transferred into the magnetosphere-ionosphere coupled system at high geomagnetic latitudes. However, there are still many fundamental questions outstanding concerning the process of merging. For example, can merging be a steady-state process or is transient merging dominant (Lockwood et al., 1993; Rodger and Dudeney, 1993)? To what extent do pressure variations within the magnetosheath precondition the magnetopause for merging to occur (Sibeck and Newell, 1992). What controls the location of merging on the magnetopause? What are the ionospheric consequences of the energetic ion and electron precipitation, and how much of the cusp currents are carried by them? Is it possible to predict the rate of change of energy transfer across the magnetopause? Can a consensus be reached for an observational and theoretical description of the ionospheric signature of flux transfer events (e.g. Scholer, 1988; Southwood et al., 1988; Fu and Lee, 1986; Lanzerotti et al., 1987; Pinnock et al., 1993)? Magnetic field oscillations over a wide range of periods can provide critical diagnostic signatures of dynamical and plasma physics processes in the coupled geospace system. For example, Pc3 magnetic pulsations are thought to be caused by wave-particle interactions up-stream of the bow shock, but how, when and why they cross the magnetosheath, and penetrate the magnetopause is not understood. How are travelling convection vortices related to magnetopause processes? Do they only contribute a tiny fraction of the energy into the ionosphere/thermosphere as suggested by Rodger and Dudeney (1993)? How important is the viscous interaction in the boundary layer in establishing ionospheric plasma convection? What is the nature of the ionospheric plasma convection pattern for northward IMF, and how does the pattern respond when the IMF changes polarity? Particle, magnetic and electric field, and imaging data from the WIND and POLAR spacecraft combined with data from SESAME and other GGS ground-based experiments should provide enough spatial and temporal coverage to address these key questions.

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