Remarkable symmetries in the Milky Way disk’s magnetic field

We report new, remarkably coherent patterns of Faraday rotation (RM) at b ∼ 15 ◦ in the inner Galactic plane, using an expanded extragalactic source RM compilation of unprecedented average accuracy. The patterns, relevant to RM pathlengths toward the inner Galactic disk, clearly indicate a global disk magnetic field structure, with remarkable reflection symmetry in RM. Sharply defined RM(l) features replicate with the opposite sign on opposite sides of the Galactic center, confirming the bi-symmetric magnetic field pattern inward of the Sagittarius-Carina arm originally found by Simard-Normandin and Kronberg (1979). The prevailing magnetic field points to l = 79± 2◦, very close to the general spiral arm direction. Additional sharp RM sign reversals in b, just below the Galactic plane, are consistent with an A0 galactic dynamo configuration. At all “outer” longitudes, there are no large scale RM sign reversals either at the Galactic plane or near to it. The outer Galactic field pattern is thus globally axisymmetric, consistent with that seen in many external spiral galaxies. Our Galaxy thus contains a mix of an axisymmetric (outer) and bisymmetric-like (inner) field configurations and would appear highly “patterned” to an extragalactic observer. This fact is of particular significance for interpretation of the “energy/species/arrival directions” of ultra high energy cosmic rays (UHECR).