Global Simulations of the Response of the Cusp to Rapid Northward Turnings of the IMF

Ion dispersions observed by the Cluster spacecraft as they cross the polar cusps offer a unique opportunity to investigate the large-scale topology and dynamics of magnetic reconnection at the dayside magnetosphere. In particular, consecutive crossings of the cusp resulting from the string of pearl configuration of the Cluster spacecraft in that region are particularly well suited for investigating the temporal evolution as well as the spatial extent of large-scale structures occurring in ion dispersions as solar wind discontinuities interact with the dayside magnetopause. We present the results of several simulation studies based on Cluster observations of ion dispersions following rapid northward turnings of the interplanetary magnetic field (IMF). First, we use three-dimensional magnetohydrodynamic (MHD) simulations to determine the global topology of the magnetic field at different times during the events. Subsequently, the time-dependent electric and magnetic fields predicted by the MHD simulations are used to compute the trajectories of a large sample of solar wind ions and to reconstruct ion dispersions at the locations of the Cluster spacecraft. Comparisons of the simulation results with Cluster ion measurements are then used to assess the accuracy of the model and to discuss the consequences of the IMF rotation on the large-scale topology and dynamics of the reconnection process and particle entry into the dayside magnetosphere.