  The new generation of low-frequency radio telescopes, such as the Low
Frequency Array (LOFAR: a Square Kilometre Array-low pathfinder), provides
advancements in our capability of probing Galactic magnetism through
low-frequency polarimetry. Maps of diffuse polarized radio emission and Faraday
rotation can be used to infer properties of, and trace structure in, the
magnetic fields in the ISM. However, to date very little of the sky has been
probed at high angular and Faraday depth resolution. We observed a 5x5 degree
region centred on the nearby galaxy IC342 using LOFAR in the frequency range
115-178 MHz at 4 arcmin resolution and performed Faraday tomography to detect
foreground Galactic polarized synchrotron emission separated by Faraday depth
(different amounts of Faraday rotation). Our Faraday depth cube shows rich
polarized structure, with up to 30 K of polarized emission at 150 MHz. We
detect two overlapping diffuse polarized features that are clearly separated in
Faraday depth. Faraday-thick structures at such low frequencies would be too
strongly depolarized to explain the observations and are therefore rejected.
Only Faraday thin structures will not be strongly depolarized; producing such
structures requires localized variations in the ratio of synchrotron emissivity
to Faraday depth per unit distance, which can arise from several physical
phenomena, such as a transition between regions of ionized and neutral gas. We
conclude that the observed polarized emission is Faraday thin, and propose that
the emission originates from two neutral clouds in the local ISM. We have
modeled the Faraday rotation for this line of sight and estimated that the line
of sight component of magnetic field of the local ISM for this direction varies
between -0.86 and +0.12 uG. We propose that this may be a useful method for
mapping magnetic fields within the local ISM.

Astrophysics of Galaxies