Interaction between magnetic topology and plasma dynamics in the quasi-steady solar wind-magnetosphere system under obliquely northward IMF conditions – Basic idea and magnetic field structure of the system –

Abstract. Conventional explanations for the formation of the solar wind–magnetosphere structure assume that the magnetic-field configuration is determined primarily by force balance and that reconnection subsequently occurs within the local topology formed by the IMF and the magnetospheric magnetic field. By contrast, because the vacuum magnetic field provides the background topology for global reconnection, force balance and the topology of the vacuum magnetic field should be regarded as playing equally important roles in determining the solar wind–magnetosphere structure. To elucidate how magnetic topology influences magnetic-field deformation and the resulting plasma dynamics, we investigate global magnetohydrodynamic (MHD) simulations that describe the behavior of a supersonic plasma flow in a background vacuum magnetic field characterized by the topology. In particular, we re-examine the MHD process from the perspective of the interaction between plasma dynamics and the deformation of the background magnetic field, and reinterpret the simulation results in the obliquely northward IMF conditions accordingly. It should be noted that the vacuum magnetic field represents the lowest magnetic-energy state. During the interaction, the force exerted by the plasma on the magnetic field is balanced by the opposing Lorentz force of the deformed magnetic field. The Lorentz force consists of the restoring force associated with deformation of the vacuum magnetic field and the magnetic pressure exerted by the magnetic flux transported into the magnetosphere through global magnetic reconnection. This force balance is referred to as the mechanical principle. Furthermore, because the two-null, two-separator structure is generally conserved and the positions of the null points remain nearly fixed in the obliquely northward IMF conditions, the magnetic field deforms subject to this topological constraint. This property is referred to as the topology-conservation property. This paper focuses on the global magnetic-field structure of the solar wind–magnetosphere system governed by these two fundamental physical laws. The main results are as follows. (1) The open magnetotail field lines that extend from a null point toward the nightside are stretched antisunward by the solar-wind flow in the near-Earth region, whereas in the distant magnetotail they gradually bend toward the IMF direction owing to the restoring force associated with the vacuum magnetic field. (2) The open field lines that extend from one null point toward the dayside separator and pass through the other null point remain directly connected to the IMF owing to the topology conservation property. Consequently, the dayside part of the cylinder undergoes little deformation, thereby causing the cylinder opening to become highly elongated and form a narrow slit-like structure. (3) The nightside separator is stretched in the antisunward direction because the two cylinders formed by the open field lines deform so as to overlap. This deformation indicates that the closed-field-line region extends into the magnetotail. Thus, the plasma sheet is formed.