pc}$ and a rms flow speed of $30~{\rm km/s}$ based on H$\alpha$ observations [@hil08]. Then, Faraday depth is calculated with the above models.

Over the frequencies we are considering ($\sim 100~{\rm MHz}-10~{\rm GHz}$), polarized emissions of galaxies mostly comes from synchrotron radiation of cosmic-ray electrons, and other radiation mechanisms can be safely neglected. This means that we need information of a cosmic-ray electron density in addition to magnetic fields derived from the above model. For it, we utilize an exponential model based on observations [@sun08]. With the cosmic-ray electron density and the amplitude and direction of magnetic fields, synchrotron emissivities and specific Stokes parameters are calculated following Equations (3-5) of [@wae09]. In this paper, we calculate the Stokes parameters at 1 GHz.

Calculation {#subsection:2.3}
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We suppose to observe a face-on spiral galaxy and use Cartesian coordinates $(x,y,z)$, where the $x-y$ plane coincides with the galactic mid-plane with $y$ pointing the anti-galactic center and $z$ penetrates the mid plane. The LOS is set to be parallel to $z$ axis, and the field-of-view center directs to $(x,y)=(0,8.5)$ in kpc (the location of the Earth in the Milky Way). We only consider the computational region of $-0.25 < x < 0.25$, $8.25 