cross the magnetic field due to the $\boldsymbol{F}_{\mathrm {rad}} \times \boldsymbol{B}$ drift. In order to understand how modification of the magnetic field lines affects the structure of the magnetosphere, we have also performed the simulation with the magnetic field fixed to the dipole field. In the case of the dipole field, the magnetic flux crossing the equatorial plane beyond $R_{\mathrm L}$ and $10 R_{\mathrm L}$ are $\Psi_{\mathrm L}=0.1$ and $\Psi_{\mathrm 10L}=0.01$, respectively. In contrast, in the case of the modified magnetic field, the values are $\Psi_{\mathrm L} \sim 0.14$ and $\Psi_{\mathrm 10L} \sim 0.03$, respectively. The closed magnetic fields are pushed out and the magnetic flux passing through the outer boundary is increased. This indicates that the open flux is increased. The top right panel of figure \[fig:res01\] shows the particle distribution and velocity field in the case of the dipole field. In comparison with the dipole case, it was found that the circulating flow diminishes in the case of the modified magnetic field. As for the total number of particles going through the outer boundary, it is $\sim 3.3 \times 10^4$ per rotation, which corresponds to $\sim 0.6 \dot{N}_{\mathrm {GJ}}$, where $\dot{N}_{\mathrm {GJ}} = (\Omega^2 R^3 B_* ) / 