Remarkable symmetries in the Milky Way disk's magnetic field
  Using a new, expanded compilation of extragalactic source Faraday rotation
measures (RM) we investigate the broad underlying magnetic structure of the
Galactic disk at latitudes $|b|$ $\lesssim 15^{\circ}$ over all longitudes $l$,
where our total number of RM's in this low-latitude range of the Galactic sky
is comparable to those in the combined Canadian Galactic Plane Survey(CGPS) at
$|b| < 4^{\circ}$ and the Southern Galactic Plane (SGPS) $|b| < 1.5^{\circ}$
survey. We report newly revealed, remarkably coherent patterns of RM at $|b|$
$\lesssim 15^{\circ}$ from $l \sim 270^{\circ}$ to $\sim 90^\circ$ 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 RM's at
$|b| \lesssim 3^{\circ}$ ("mid-plane") and our new RM's up to $|b| \sim
15^{\circ}$ ("near-plane").
  Our analysis also shows the approximate $z$-height -- the vertical height of
the coherent component of the disc field above the Galactic disc's mid-plane --
to be $\sim 1.5$kpc out to $\sim 6$ kpc from the Sun. This identifies the
approximate height of the transition layer to the halo field structure. We find
no RM sign change across the plane within $|b| \sim 15^{\circ}$ in any
longitude range. The prevailing {\it 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 $5.5^{\circ}\, \pm 1^{\circ}$ at all $|b|$ up to $\sim 12^{\circ}$ in
the inner semicircle of Galactic longitudes. It decreases to $\sim 0^{\circ}$
toward the anticentre.
