Large Scale Structure of Magnetic Field, Plasma and Currents in the Magnetosphere

In this study we present fully 3D quasi-static equilibrium solutions of the magnetosphere by using observed plasma pressure distributions and boundary conditions based on the Tsyganenko '96 magnetospheric field model. The pressure profiles we use are obtained from several spacecraft: data from ISEE and AMPTE/CCE is used for specifying a pressure profile on the midnight equatorial line; pressure values obtained by DMSP are taken at the ionosphere and then mapped on the computed field lines throughout the magnetosphere; GEOTAIL pressure data in the plasma sheet is used. The magnetospheric magnetic field in our model is conveniently described in terms of Euler potentials (in particular, we can also obtain a set of Euler potentials for the Tsyganenko magnetic field). Having the magnetic field in this representation provides a great advantage for applicative purposes [Stern, JGR, 1994]. The solutions are numerically computed in the closed-field as well as in the open field regions of the magnetosphere using an improved version of our MHD code MAG-3D [Cheng, GRL, 1995], and contain 3D magnetospheric magnetic field configurations and current system including the ring current, cross-tail current and field-aligned current distributions. We discuss the change of the field and current structures under different disturbed conditions through changes in plasma pressure distribution as well as in the boundary conditions. In particular, we discuss the appearance of thin cross-tail current sheets during disturbed times, and we look at how the Birkeland currents respond to changes in the solar wind pressure and IMF.