  With the Westerbork Synthesis Radio Telescope (WSRT), multi-frequency
polarimetric images were taken of the diffuse radio synchrotron background in a
region centered on (l,b) = (161,16). The observations were done simultaneously
in 5 frequency bands from 341 MHz to 375 MHz, with 5 arcmin resolution.
Ubiquitous structure on arcminute and degree scales in the polarized intensity
and polarization angle, combined with no observed structure in total intensity,
indicates that the structure in the polarized radiation must be due to Faraday
rotation and depolarization mostly in the warm nearby Galactic interstellar
medium (ISM). Beam depolarization most likely creates "depolarization canals"
of one beam wide, while depth depolarization is responsible for creating most
of the structure on scales larger than a beam width. Rotation measures RM are
in the range -17 < RM < 10 rad/m2 with a non-zero average of about -3.4 rad/m2.
The gradient and average RM are consistent with a regular magnetic field of
about 1 uG which has a pitch angle of p = -14 degrees. 13 Extragalactic sources
in the field have |RM| < 13 rad/m2, with an estimated intrinsic source
contribution of 3.6 rad/m2. The RMs of the extragalactic sources show a
gradient (with a sign reversal) that is about 3 times larger than the gradient
in the RMs of the diffuse emission, and that is approximately in Galactic
latitude. This difference is ascribed to a vastly different effective length of
the line of sight. The observations are interpreted in terms of a
single-cell-size model of the warm ISM which contains gas and magnetic fields,
with a polarized background.
