Simulation on ring current formation: A case study of a storm on February 13, 1972

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We developed a computer simulation scheme for the motion of energetic particles trapped in the Earth's magnetic field during a magnetic storm. The particles are injected into the inner magnetosphere from the near-Earth plasma sheet by a strong inductive electric field due to a dipolarization in association with a substorm. The motion of the particles is traced by the bounce-average approximation under a dipolar magnetic field and the Volland-Stern type convection field. The absolute quantity of a directional differential flux of the particles can be obtained. We can calculate the azimuthal component of the current density and an H-component of the magnetic disturbance at the center of the Earth (Dst^*) induced by the current. In this paper, we examine the formation of the ring current and the magnetic disturbance on the moderate storm occurred on February 13-14,1972. We find that (1) the inductive electric field due to the dipolarization may be one of major contributors to Dst^* on this storm, (2) the steep variations of Dst^* in the main phase cannot reproduced by the inductive electric field model, (3) the intensity of Dst^* is sensitive to the number density in the near-Earth plasma sheet, that is, an enhancement of the number density in the near-Earth plasma sheet may produce the steep and the major variations of Dst^*.
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