Diffuse polarized emission in the LOFAR Two-meter Sky Survey
  Faraday tomography allows us to map diffuse polarized synchrotron emission
from our Galaxy and use it to interpret the magnetic field in the interstellar
medium (ISM). We have applied Faraday tomography to 60 observations from the
LOFAR Two-meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic
covering 568 square degrees at high Galactic latitudes, at 4.3' angular
resolution and 1 rad m$^{-2}$ Faraday depth resolution, with a typical noise
level of 50--100 $\mu$Jy per point spread function (PSF) per rotation measure
spread function (RMSF) (40-80 mK RMSF$^{-1}$). While parts of the images are
strongly affected by instrumental polarization, we observe diffuse polarized
emission throughout most of the field, with typical brightness between 1 and 6
K RMSF$^{-1}$, and Faraday depths between $-7$ and +25 rad m$^{-2}$.
  We observed many new polarization features, some up to 15 degrees in length.
These include two regions with very uniformly structured, linear gradients in
the Faraday depth; we measured the steepness of these gradients as 2.6 and 13
rad m$^{-2}$ deg$^{-1}$. We also observed a relationship between one of the
gradients and an HI filament in the local ISM. Other ISM tracers were also
checked for correlations with our polarization data and none were found, but
very little signal was seen in most tracers in this region. We conclude that
the LOTSS data are very well suited for Faraday tomography, and that a
full-scale survey with all the LOTSS data has the potential to reveal many new
Galactic polarization features and map out diffuse Faraday depth structure
across the entire northern hemisphere.
