Magnetospheric response to solar wind pressure during strongly southward IMF: MHD simulations

The solar wind energy input to the magnetosphere is generally controlled by the solar wind magnetic field, with significant energy transfer occurring during periods of southward IMF. In fact, empirical expressions of the energy coupling tend to be functions of solar wind speed, magnetic field, and the orientation of the field, but do not include the density or dynamic pressure as a factor. Recent investigations, both observational and theoretical, have suggested that the dynamic pressure can play a significant role in coupling solar wind energy to the magnetosphere during periods of strongly southward IMF. In the paper, we present results of global MHD models using idealized solar wind conditions as controlled experiments. We find that for moderate driving levels there is little influence of dynamic pressure, but during strong IMF driving the pressure is a major, or even dominant, factor in controlling the dayside merging rate and magnetospheric energy dissipation. We will discuss these results in the context of determining the geoeffectiveness of extreme solar wind conditions like magnetic clouds arising from CMEs.