Remarkable Symmetries in the Milky Way Disc's Magnetic Field
P. P. Kronberg A B E and K. J. Newton-McGee C DA Department of Physics, University of Toronto, Toronto, M5S 1A7, Canada
B IGPP, Los Alamos National Laboratory, M.S. T006, Los Alamos, NM 87545, USA
C Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
D Australia National Telescope Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia
E Corresponding author. Email: kronberg@physics.utoronto.ca
Publications of the Astronomical Society of Australia 28(2) 171-176 https://doi.org/10.1071/AS10045
Submitted: 29 November 10 Accepted: 4 March 11 Published: 22 June 2011
Journal Compilation © Astronomical Society of Australia 2011
Abstract
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 ~1.5 kpc out to ~6 kpc 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.
Keywords: magnetic fields — methods: observational — Galaxy: disc — Galaxy: structure — radio continuum: ISM
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