Acceleration of ionospheric out-flowing ions in the substorm in geo-magnetotail

Using dynamics equation, acceleration of out-flowing ion during dipolarization in a substorm in the magnetotail is simulated. The main results show that: (1) The ion distribution function that is initially exponentially decreasing with increasing speed is turned into a single peak distribution, and with time the peak moves towards higher speed. (2) The peak moves along V_⊥ faster than that along V_∥, and the ion acceleration mainly occurs in the middle of the dipolarization. (3) The higher the initial energy, the faster the peak moves, and the more energy is obtained by the ions. The ion energy theoretically calculated is as high as about 10² keV, this is consistent with the observation.