Fundamental physical processes of the steady solar wind-magnetosphere system under northward IMF conditions in the framework of the magnetic topology

To understand the physical processes of the steady solar wind-magnetosphere system, two aspects must be considered: (i) the dynamical processes, which govern the distribution of mass, momentum, and energy, and (ii) the magnetic field topology, which governs the three-dimensional reconnection between the solar wind and the magnetosphere. Because the magnetic topology is determined by the combined interplanetary magnetic field (IMF) and geomagnetic field, the solar wind and magnetosphere should be treated as a single magnetohydrodynamic (MHD) fluid system. In this unified system, the physical processes can be interpreted as the interaction between the plasma and the magnetic field. When the plasma is absent, a vacuum magnetic-field configuration emerges, representing the system's ground state. Therefore, the dynamics of the system can be described as a balance between two forces: a force returning the magnetic field to its ground state and a force exerted by the solar wind plasma that deforms the magnetic field lines. This framework is referred to as the mechanical principle. The vacuum magnetic field exhibits a characteristic topology with two null points and two separators, which provide the magnetic framework for separator reconnection. Global MHD simulations have confirmed that this topology is preserved under northward IMF conditions, a property we refer to as the topology conservation property. Both the mechanical principle and the topology conservation property together determine the magnetic field structure of a quasi-steady solar wind-magnetosphere system. Therefore, this study achieves a fundamental understanding of the interaction between magnetic topology and plasma dynamics in the solar wind-magnetosphere system in the northward IMF conditions. Within this framework, we discussed that both the topology conservation property and the mechanical principle play essential roles in the formation of the steady-state magnetic field structure of the magnetotail and the plasma sheet in the northward IMF condition.