  We present a study of synthetic observations of polarized dust emission at
353 GHz as seen by an observer within a cavity in the interstellar medium
(ISM). The cavity is selected from a magnetohydrodynamic simulation of the
local ISM with time-dependent chemistry, star formation, and stellar feedback
in form of supernova explosions with physical properties comparable to the
Local Bubble ones. We find that the local density enhancement together with the
coherent magnetic field in the cavity walls makes the selected candidate a
translucent polarization filter to the emission coming from beyond its domains.
This underlines the importance of studying the Local Bubble in further detail.
The magnetic field lines inferred from synthetic dust polarization data are
qualitatively in agreement with the all-sky maps of polarized emission at 353
GHz from the Planck satellite in the latitudes interval 15deg <= |b| <= 65deg.
As our numerical simulation allows us to track the Galactic midplane only out
to distances of 250 pc, we exclude the region |b|<15deg from our analysis. At
large Galactic latitudes, our model exhibits a high degree of small-scale
structures. On the contrary, the observed polarization pattern around the
Galactic poles is relatively coherent and regular, and we argue that the global
toroidal magnetic field of the Milky Way is important for explaining the data
at |b| > 65deg. We show that from our synthetic polarization maps, it is
difficult to distinguish between an open and a closed Galactic cap using the
inferred magnetic field morphology alone.
