Local measurements of the mean interstellar polarization at high
  Galactic latitudes
  We conduct a small-scale pathfinding survey designed to identify the average
polarization properties of the diffuse ISM locally at the lowest dust content
regions. We perform deep optopolarimetric surveys within three $\sim 15' \times
15'$ regions located at $b > 48^\circ$, using the RoboPol instrument. The
observed samples of stars are photometrically complete to $\sim$16 mag in the
R-band. The selected regions exhibit low dust emission at 353 GHz and low total
reddening compared to the majority of high-latitude sightlines. We measure the
level of systematic uncertainty for all observing epochs and find it to be
0.1\% in fractional linear polarization, $p$. The majority of individual
stellar measurements are non-detections. However, our survey strategy enables
us to locate the mean fractional linear polarization $p_{mean}$ in each of the
three regions. The region with lowest dust content yields $p_{mean}=(0.054 \pm
0.038) \%$, not significantly different from zero. We find significant
detections for the remaining two regions of: $p_{mean}=(0.113 \pm 0.036) \%$
and $p_{mean}=(0.208 \pm 0.044) \%$. Using a Bayesian approach we provide upper
limits on the intrinsic spread of the small-scale distributions of $q$ and $u$.
At the detected $p_{mean}$ levels, the determination of the systematic
uncertainty is critical for the reliability of the measurements. We verify the
significance of our detections with statistical tests, accounting for all
sources of uncertainty. Using publicly available HI emission data, we identify
the velocity components that most likely account for the observed $p_{mean}$
and find their morphologies to be misaligned with the orientation of the mean
plane-of-sky magnetic field at a spatial resolution of 10$\arcmin$. We find
indications that the standard upper envelope of $p$ with reddening
underestimates the maximum $p$ at very low E(B-V) ($\leq 0.01$ mag).
