Poisson equation, are summarized as follows. Most of the PC and SG models assume $E_{||}=0$, as well as $\Phi=0$ at the stellar surface, and $\Phi=0$ along the last open field line. In addition, the condition $E_{||}=0$ is imposed at an upper boundary, ie, PFF. The OG models, which elongate along open field lines, usually have upper and lower boundaries along the field lines; an inner boundary, which is on the star side; and an outer boundary, which is located near the light cylinder. In particular, the lower boundary is supposed to be the last open field line. A standard boundary condition is such that $\Phi=0$ on the upper and lower boundaries, $\Phi=0$ and $E_{||}=0$ on the inner boundary, and $E_{||}=0$ on the outer boundary. The boundary conditions are very important because they affect the structure of the accelerating electric field. However, the screening process on each boundary has not been well modeled or simulated. Therefore, it is not clear whether $\Phi=0$ along the last open field line or not.

There is also a problem with the directions of the current flow. The directions of the current along the magnetic field line above the last open field line are different in 