NUMERICAL STUDY ON THE EARTH'S MAGNETOTAIL FORMATION AND MAGNETOSPHERIC SUBSTORM (RECONNECTION).

摘要

The formation of the magnetotail configuration under the influence of the solar wind plasma flow entering the magnetosphere and the associated magnetospheric substorm in the magnetotail region are studied numerically by means of time dependent nonlinear resistive MHD codes. By using the two dimensional model, the formation of the magnetotail starting from a dipole magnetic field is studied. It is observed that a two dimensional tail-like structure develops in association with the plasma flow originating from the solar wind. In the presence of a current driven resistivity, the x-type reconnection is triggered at the point where the current density is peaked. The plasma sheet width does not change much after reconnection in contrast to the pre-reconnection phase in which the plasma sheet becomes thinner and thinner. Formation of the plasmoid and its tailward motion are also observed. The temporal behavior of the system is very sensitive to the solar wind energy flux which may originate from the dayside reconnection. When the solar wind energy flux is larger, the development of magnetic reconnection is found to be faster, and the resulting plasma motions are more violent. The x-point is found to be formed closer to the earth in that case. The substorm recovery phase is studied by reducing the entering solar wind energy flux when magnetic reconnection is fully developed. The system is found to respond instantly to this change. The magnetic x-point moves tailward and the field line configuration is restored back to a tail-like one in the earth side part of the x-point. By using the three dimensional model, the generation of the field aligned currents is studied. The field aligned currents are found to be generated in the earth side region of the magnetic x-point when magnetic reconnection fully develops, which reduces the cross tail currents in the midnight region of the earthward part of the x-point. The currents have the sense of the region 1 current, which suggests that the region 1 current in the deep tail is due to the diversion of the neutral sheet current.