A numerical study of solar wind - magnetosphere interaction for northward interplanetary magnetic field

The solar wind-magnetosphere interaction for northward interplanetary magne tic field (IMF) is studied using a newly developed three-dimensional adapti ve mesh refinement (AMR) global MHD simulation model. The simulations show that for northward IMF the magnetosphere is essentially closed. Reconnectio n between the IMF and magnetospheric field is limited to finite regions nea r the cusps. When the reconnection process forms newly closed magnetic fiel d lines on the dayside, the solar wind plasma trapped on these reconnected magnetic field lines becomes part of the low-latitude boundary layer (LLBL) plasma and it convects to the nightside along the magnetopause. The last c losed magnetic field line marks the topological boundary of the magnetosphe ric domain. When the last closed magnetic field line disconnects at the cus ps and reconnects to the IMF, its plasma content becomes part of the solar wind. Plasma convection in the outer magnetosphere does not directly contri bute to the reconnection process. On the dayside the topological boundary b etween the solar wind and the magnetosphere is located at the inner edge of the magnetopause current layer. At the same time, multiple current layers are observed in the high-altitude cusp region. Our convergence study and di agnostic analysis indicate that the details of the diffusion and the viscou s interaction do not play a significant role in controlling the large-scale configuration of the simulated magnetosphere. It is sufficient that these dissipation mechanisms exist in the simulations. In our series of simulatio ns the length of the magnetotail is primarily determined by the balance bet ween the boundary layer driving forces and the drag forces. With a parametr ic study, we find that the tail length is proportional to the magnetosheath plasma beta near the magnetopause at local noon. A higher solar wind densi ty, weaker IMF, and larger solar wind Mach number results in a longer tail. On the nightside downstream of the last dosed magnetic field line the plas ma characteristics are similar to that in the magnetotail, posing an observ ational challenge for identification of the topological status of the corre sponding field lines.