a Mach number, the plasma beta, the driving scale, and so on, at mid Galactic latitudes are necessary.

Let us discuss the observational prospect for galactic FDFs. In the past studies, the shape of galactic FDFs has often been assumed to be delta function, top-hat or gaussian function. However, we saw that realistic FDFs are not so simple and even a single galaxy has multiple peaks. Furthermore, it is common to assume the intrinsic polarization angle does not change within a single source, which is not obviously reasonable. These issues are particularly important when we use QU-fitting method [@osu12; @ide14], where we need to assume a specific functional form of FDF and fit it with the data, $P(\lambda^2)$, to obtain the parameter values. We need further studies to find appropriate functions to be fitted.

On the other hand, with RM synthesis, we can directly reconstruct the FDF from the data without any assumption on its form. However, the reconstruction is not perfect because $P(\lambda^2)$ is not physical for negative $\lambda^2$ and is observationally limited even for positive $\lambda^2$. Especially, it was shown that multiple peaks tend to induce false signal in FDF [@far11; @kum14]. It is important to study how 