  Magnetic fields in the turbulent interstellar medium (ISM) are a key element
in understanding Galactic dynamics, but there are many observational
challenges. One useful probe for studying the magnetic field component parallel
to the line of sight (LoS) is Faraday rotation of linearly polarized radio
synchrotron emission, combined with H$\alpha$ observations. HII regions are the
perfect laboratories to probe such magnetic fields as they are localized in
space, and are well-defined sources often with known distances and measurable
electron densities. We chose the HII region Sharpless 2-27 (Sh 2-27). By using
a map of the magnetic field strength along the LoS ($B_{\parallel}$) for the
first time, we investigate the basic statistical properties of the turbulent
magnetic field inside Sh 2-27. We study the scaling of the magnetic field
fluctuations, compare it to the Kolmogorov scaling, and attempt to find an
outer scale of the turbulent magnetic field fluctuations. We estimate the
median value of $n_e$ as $7.3\pm0.1$ cm$^{-3}$, and the median value of
$B_{\parallel}$ as $-4.5\pm0.1$ $\mu$G, which is comparable to the magnetic
field strength in diffuse ISM. The slope of the structure function of the
estimated $B_{\parallel}$-map is found to be slightly steeper than Kolmogorov,
consistent with our Gaussian-random-field $B_{\parallel}$ simulations revealing
that an input Kolmogorov slope in the magnetic field results in a somewhat
steeper slope in $B_{\parallel}$. These results suggest that the lower limit to
the outer scale of turbulence is 10 pc in the HII region, which is comparable
to the size of the computation domain. This may indicate that the turbulence
probed here could actually be cascading from the larger scales in the ambient
medium, associated with the interstellar turbulence in the general ISM, which
is illuminated by the presence of Sh 2-27.
