This is why the beam depolarization is not so significant. It would be interesting to extend our calculation to larger regions to study the possibility of probing the global topology of magnetic fields by combining the Faraday tomography and imaging. Remembering that FDFs are contributed appreciably from the halo region, as well as the disk, halo fields would also be able to be probed. For instance, the three peaks seen in the FDF would not be expected if there are no halo toroidal magnetic field, although the three peaks do not always appear in the FDF even if there are halo toroidal magnetic field.

We also compared the cases between axi-symmetric and bi-symmetric spiral fields, and the cases between the dipole poloidal field and the X-field [@arkg13], but we did not see notable differences on the resultant FDFs. This may be due to the fact that we calculated FDFs for a relatively small region up to 500 pc $\times$ 500 pc. A study for probing the global topology of magnetic fields could be possible by using global MHD simulations of galactic gaseous disk [@mac13] or by improving the model we adopted. For the improvement, further investigations of turbulence such as 