Polarized synchrotron emission is the Galaxy's magnetic field made visible: relativistic electrons spiraling along field lines emit radiation whose polarization angle traces the field orientation, and whose degree of polarization measures the field's order. Along the Galactic plane, WMAP and Planck find that order missing: the synchrotron emission is far more depolarized than standard Faraday rotation and turbulence models predict (Planck Collaboration 2016).
The standard depolarization inventory falls short. Beam depolarization, unresolved angle structure within the telescope beam, and depth depolarization, the averaging of rotated polarization angles along the line of sight, are both modeled and both insufficient: a residual depolarization remains that requires either small-scale magnetic turbulence at levels beyond what magnetohydrodynamic models of the interstellar medium produce, or a distinct Faraday-thick component of ionized, magnetized gas that current foreground templates simply do not contain (Pasetto et al. 2018). Either option says the magnetized medium has structure, in field topology or in ionized-gas distribution, that the statistically uniform models miss.
The standing joins the wider foreground bottleneck: depolarization that cannot be modeled propagates directly into errors in the polarized foreground templates that B-mode cosmology depends on, making the plane's missing polarization a practical problem far beyond Galactic astronomy.