  The W50/SS433 system is an unusual Galactic outflow-driven object of
debatable origin. We have used the Australia Telescope Compact Array (ATCA) to
observe a new 198 pointing mosaic, covering $3^\circ \times 2^\circ$, and
present the highest-sensitivity full-Stokes data of W50 to date using
wide-field, wide-band imaging over a 2 GHz bandwidth centred at 2.1 GHz. We
also present a complementary H$\alpha$ mosaic created using the Isaac Newton
Telescope Photometric H$\alpha$ Survey of the Northern Galactic Plane (IPHAS).
The magnetic structure of W50 is found to be consistent with the prevailing
hypothesis that the nebula is a reanimated shell-like supernova remnant (SNR),
that has been re-energised by the jets from SS433. We observe strong
depolarization effects that correlate with diffuse H$\alpha$ emission, likely
due to spatially-varying Faraday rotation measure (RM) fluctuations of $\ge48$
to 61 rad m$^{-2}$ on scales $\le4.5$ to 6 pc. We also report the discovery of
numerous, faint, H$\alpha$ filaments that are unambiguously associated with the
central region of W50. These thin filaments are suggestive of a SNR's shock
emission, and almost all have a radio counterpart. Furthermore, an RM-gradient
is detected across the central region of W50, which we interpret as a loop
magnetic field with a symmetry axis offset by $\approx90^{\circ}$ to the
east-west jet-alignment axis, and implying that the evolutionary processes of
both the jets and the SNR must be coupled. A separate RM-gradient is associated
with the termination shock in the Eastern ear, which we interpret as a
ring-like field located where the shock of the jet interacts with the
circumstellar medium. Future optical observations will be able to use the new
H$\alpha$ filaments to probe the kinematics of the shell of W50, potentially
allowing for a definitive experiment on W50's formation history.
