Ion Accelleration in the Magnetotail during Magnetic Dipolarazation: I Temporal Variation of Ion Distribution Fuction

Using dynamics equation, the temporal variation of ion distribution function during dipolarization of a substorm in the magneto-tail is simulated. The main results show that: (1) The ion distribution function which is initially exponentially decreasing with increasing speed becomes a single-peak distribution both in the parallel direction and in the perpendicular direction, and the peak moves towards higher speed with time. (2) In the phase space, the peak moves faster along V┴ direction than along V// direction. (3) The peak of lighter ion moves faster than that of heavier ion. (4) The higher the ion's initial energy, the faster the peak moves. (5) During the dipolarization, the ions move as an earthward flow. The acceleration of up-flowing ions by induced electric field is discussed.