  Rotation Measure synthesis (RM synthesis) of the Westerbork Synthesis Radio
Telescope (WSRT) observations at 2 m wavelength of the FAN region at l=137deg,
b=+7deg shows the morphology of structures in the ionized interstellar medium.
We interpret the diffuse polarized synchrotron emission in terms of coherent
structures in the interstellar medium and the properties of the interstellar
magnetic field. For the first time, cross-correlation is applied to identify
and characterize polarized structures in Faraday depth space. Complementary
information about the medium are derived from H$\alpha$ emission, properties of
nearby pulsars, and optical polarized starlight measurements. Three
morphological patterns are recognized, showing structures on scales from
degrees down to the beam size. At low Faraday depth values, a low gradient
across the imaged field is detected, almost aligned with the Galactic plane.
Power spectra of polarized structures in Faraday depth space provide evidence
of turbulence. A sign reversal in Faraday depth space indicates a reversal of
the magnetic field component along the line of sight, from towards the observer
and nearby to away from the observer at larger distances. The distance to the
nearby, extended component is estimated to be lesser than 100 pc, which
suggests that this structure corresponds to the Local Bubble wall. For the
circular component, various physical interpretations are discussed. The most
likely explanation is that the circular component seems to be the presence of a
nearby (about 200 pc away) relic Stromgren sphere, associated with an old
unidentified white dwarf star and expanding in a low-density environment.
