Remarkable Symmetries in the Milky Way Disc's Magnetic Field

We apply a new, expanded compilation of extragalactic source Faraday rotation measures (RM) to investigate the broad underlying magnetic structure of the Galactic disk at latitudes |b|<~15° over all longitudes l, where our total number of RMs is comparable to those in the combined Canadian Galactic Plane Survey (CGPS) at |b|<4° and the Southern Galactic Plane (SGPS) |b|<1.5°. We report newly revealed, remarkably coherent patterns of RM at |b|<~15° from l~270° to ~90° and RM(l) features of unprecedented clarity that replicate in l with opposite sign on opposite sides of the Galactic center. They confirm a highly patterned bisymmetric field structure toward the inner disc, an axisymmetic pattern toward the outer disc, and a very close coupling between the CGPS/SGPS RMs at |b|<~3° (`mid-plane') and our new RMs up to |b|~15° (`near-plane'). Our analysis also shows the vertical height of the coherent component of the disc field above the Galactic disc's mid-plane - to be ~1.5kpc out to ~6kpc from the Sun. This identifies the approximate height of a transition layer to the halo field structure. We find no RM sign change across the plane within |b|~15° in any longitude range. The prevailing disc field pattern and its striking degree of large-scale ordering confirm that our side of the Milky Way has a very organized underlying magnetic structure, for which the inward spiral pitch angle is 5.5°+/-1° at all |b| up to ~12° in the inner semicircle of Galactic longitudes. It decreases to ~0° toward the anticentre.