17:15 〜 18:45
[PEM11-P07] Unified understanding of the dependence of the magnetosphere system on the solar wind Mach number using skeletal magnetic field concept
キーワード：磁気圏トポロジー、MHDシミュレーション、太陽風マッハ数
The warm humid air mass and the cool dry air mass in the atmosphere do not mix, and a front is formed between them. The cyclone on the front line plays a role in mixing both air masses. On the other hand, plasmas in the solar wind and the magnetosphere do not mix, and a boundary, the magnetopause, is formed between them. The reconnection plays a role in mixing the plasmas. So, a similar appearance between the magnetopause-reconnection system in space science and the front-cyclone system of meteorology suggests that the solar wind-magnetosphere system can be treated as a single system like the Earth’s atmospheric system.
What does it mean to consider the solar wind magnetosphere system as a single system? The answer is that, instead of studying the interaction between two magnetized plasmas, - the solar wind and the magnetosphere -, we can think of them as a single MHD fluid. This means that we only need to study the interaction between magnetic fields and plasma to manifest the dynamics in the solar wind-magnetosphere system. As for the magnetic field in the solar wind-magnetosphere single system, we already know that it exhibits the null-separator structure with a torus and cylinders when plasmas are removed from the solar wind-magnetosphere system [e.g., Lau and Fin, 1990]. This field structure is a superposition of the Earth’s dipole field and uniform IMF field. As a result, to study the solar wind-magnetosphere system as a single system is to manifest the deformation of this magnetic field structure by plasmas (We also proved that the topology of the magnetic field, -torus and cylinder- remains in the plasma environment).
In addition, we note that this superposition does not have free energy (in other words no magnetic tension). The free energy (the magnetic tension) is caused by the deformation of the superposition by plasmas. Consequently, the deformation of the magnetosphere system can be understood in a unified way by the interplay between the deforming force exerted by the plasmas in the solar wind and magnetosphere and the restoring force arising from the magnetic tension toward the superposition. From the above discussion, the superposition can be called a skeletal magnetic field structure.
How does magnetic reconnection operate? A null point is inherently present in the skeletal magnetic field. Therefore, there is no need to create a null point by the kinetic action of plasma. (This situation is different from the tail reconnection at the onset of a substorm.) Consequently, the fundamental magnetic field structure remains largely unaltered due to the kinetic effects of plasma. Specifically, the overdraping magnetic field observed under northward IMF conditions is pre-existing within the skeletal magnetic field. However, the MHD condition does not hold near the null point due to the weak magnetic field. Consequently, the magnetic field lines transported by the solar wind undergo successive reconnection with the magnetospheric magnetic field lines near the null point, facilitating the entry of plasma from the solar wind into the magnetosphere.
Here we apply this hypothesis to understand the dependence of the magnetosphere-ionosphere system on the solar wind Mach number by changing IMF intensity in the northward IMF condition. In the talk, we will present 1) the high-speed magnetosheath flow in the strong IMF condition discussed by Lavroud et al. (2007) and Lavraud and Borovsky (2008); 2) the relation between the CPCP and IMF intensity, 3) the viscous interaction for the extremely low Mach solar wind; and others.
Lau, Y.-T. and Finn, J. M. (1990), Astrophysical Journal, 350, 672-691.
Lavraud, B., J. E. Borovsky, A. J. Ridley, E. W. Pogue, M. F. Thomsen, H. Reme, A. N. Fazakerley, and E. A. Lucek (2007), Geophys. Res. Lett., 34, L14102, doi:10.1029/2007GL030024.
Lavraud, B., and J. E. Borovsky (2008), J. Geophys. Res., 113, A00B08, doi:10.1029/2008JA013192.
What does it mean to consider the solar wind magnetosphere system as a single system? The answer is that, instead of studying the interaction between two magnetized plasmas, - the solar wind and the magnetosphere -, we can think of them as a single MHD fluid. This means that we only need to study the interaction between magnetic fields and plasma to manifest the dynamics in the solar wind-magnetosphere system. As for the magnetic field in the solar wind-magnetosphere single system, we already know that it exhibits the null-separator structure with a torus and cylinders when plasmas are removed from the solar wind-magnetosphere system [e.g., Lau and Fin, 1990]. This field structure is a superposition of the Earth’s dipole field and uniform IMF field. As a result, to study the solar wind-magnetosphere system as a single system is to manifest the deformation of this magnetic field structure by plasmas (We also proved that the topology of the magnetic field, -torus and cylinder- remains in the plasma environment).
In addition, we note that this superposition does not have free energy (in other words no magnetic tension). The free energy (the magnetic tension) is caused by the deformation of the superposition by plasmas. Consequently, the deformation of the magnetosphere system can be understood in a unified way by the interplay between the deforming force exerted by the plasmas in the solar wind and magnetosphere and the restoring force arising from the magnetic tension toward the superposition. From the above discussion, the superposition can be called a skeletal magnetic field structure.
How does magnetic reconnection operate? A null point is inherently present in the skeletal magnetic field. Therefore, there is no need to create a null point by the kinetic action of plasma. (This situation is different from the tail reconnection at the onset of a substorm.) Consequently, the fundamental magnetic field structure remains largely unaltered due to the kinetic effects of plasma. Specifically, the overdraping magnetic field observed under northward IMF conditions is pre-existing within the skeletal magnetic field. However, the MHD condition does not hold near the null point due to the weak magnetic field. Consequently, the magnetic field lines transported by the solar wind undergo successive reconnection with the magnetospheric magnetic field lines near the null point, facilitating the entry of plasma from the solar wind into the magnetosphere.
Here we apply this hypothesis to understand the dependence of the magnetosphere-ionosphere system on the solar wind Mach number by changing IMF intensity in the northward IMF condition. In the talk, we will present 1) the high-speed magnetosheath flow in the strong IMF condition discussed by Lavroud et al. (2007) and Lavraud and Borovsky (2008); 2) the relation between the CPCP and IMF intensity, 3) the viscous interaction for the extremely low Mach solar wind; and others.
Lau, Y.-T. and Finn, J. M. (1990), Astrophysical Journal, 350, 672-691.
Lavraud, B., J. E. Borovsky, A. J. Ridley, E. W. Pogue, M. F. Thomsen, H. Reme, A. N. Fazakerley, and E. A. Lucek (2007), Geophys. Res. Lett., 34, L14102, doi:10.1029/2007GL030024.
Lavraud, B., and J. E. Borovsky (2008), J. Geophys. Res., 113, A00B08, doi:10.1029/2008JA013192.