Temporal variation of the distribution function and acceleration of O+ ions during substorm dipolarization

By using Vlasov's Equation, the temporal variation of the distribution function of O+ ions and the acceleration of the ions during dipolarization in a substorm in the magnetotail are simulated. The main results show that: (1) the ion distribution function which is initially exponentially decreasing with increasing velocity becomes a single peak distribution, and with time the peak moves towards higher velocity; (2) the peak moves faster along the perpendicular direction than along the parallel direction; (3) the ion acceleration occurs mainly in the middle period of the dipolarization; (4) the higher the initial energy, the faster the peak moves, and the more energy is obtained by the ions, (5) during the dipolarization, the ions move as an Earthward flow and their energy can reach an order of magnitude of 102 keV, which is consistent with some observations.