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 SNRs 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 W50s formation history.
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