Through thick or thin: Multiple components of the magneto-ionic medium
  towards the nearby ${\rm H\,{\small II}}$ region Sharpless 2-27 revealed by
  Faraday tomography
  Sharpless 2-27 (Sh2-27) is a nearby ${\rm H\,{\small II}}$ region excited by
$\zeta$Oph. We present observations of polarized radio emission from 300 to
480$\,$MHz towards Sh2-27, made with the Parkes 64$\,$m Radio Telescope as part
of the Global Magneto-Ionic Medium Survey. These observations have an angular
resolution of $1.35^{\circ}$, and the data are uniquely sensitive to
magneto-ionic structure on large angular scales. We demonstrate that background
polarized emission towards Sh2-27 is totally depolarized in our observations,
allowing us to investigate the foreground. We analyse the results of Faraday
tomography, mapping the magnetised interstellar medium along the 165$\,$pc path
to Sh2-27. The Faraday dispersion function in this direction has peaks at three
Faraday depths. We consider both Faraday thick and thin models for this
observation, finding that the thin model is preferred. We further model this as
Faraday rotation of diffuse synchrotron emission in the Local Bubble and in two
foreground neutral clouds. The Local Bubble extends for 80$\,$pc in this
direction, and we find a Faraday depth of $-0.8 \pm 0.4\,$rad$\,$m$^{-2}$. This
indicates a field directed away from the Sun with a strength of
$-2.5\pm1.2\,\mu$G. The near and far neutral clouds are each about 30$\,$pc
thick, and we find Faraday depths of $-6.6\pm0.6\,$rad$\,$m$^{-2}$ and
$+13.7\pm0.8\,$rad$\,$m$^{-2}$, respectively. We estimate that the
line-of-sight magnetic strengths in the near and far cloud are $B_{\parallel,
\text{near}} \approx -15\,\mu\text{G}$ and $B_{\parallel, \text{far}} \approx
+30\,\mu\text{G}$. Our results demonstrate that Faraday tomography can be used
to investigate the magneto-ionic properties of foreground features in front of
nearby ${\rm H\,{\small II}}$ regions.
