Interpretation of the structure function of rotation measure in the
  interstellar medium
  The observed structure function (SF) of rotation measure (RM) varies as a
broken power-law function of angular scales. The systematic shallowness of its
spectral slope is inconsistent with the standard Kolmogorov scaling. This
motivates us to examine the statistical analysis on RM fluctuations. The
correlations of RM constructed by Lazarian & Pogosyan (2016) are demonstrated
to be adequate in explaining the observed features of RM SFs through a direct
comparison between the theoretically obtained and observationally measured SF
results. By segregating the density and magnetic field fluctuations and
adopting arbitrary indices for their respective power spectra, we find that
when the SFs of RM and emission measure have a similar form over the same range
of angular scales, the statistics of the RM fluctuations reflect the properties
of density fluctuations. RM SFs can be used to evaluate the mean magnetic field
along the line of sight, but cannot serve as an informative source on the
properties of turbulent magnetic field in the interstellar medium. We identify
the spectral break of RM SFs as the inner scale of a shallow spectrum of
electron density fluctuations, which characterizes the typical size of discrete
electron density structures in the observed region.
