The Coherent Magnetic Field of the Milky Way
arxiv(2023)
Abstract
We present a suite of models of the coherent magnetic field of the Galaxy
(GMF) based on new divergence-free parametric functions describing the global
structure of the field. The model parameters are fit to the latest full-sky
Faraday rotation measures of extragalactic sources (RMs) and polarized
synchrotron intensity (PI) maps from WMAP and Planck. We employ multiple models
for the density of thermal and cosmic-ray electrons in the Galaxy, needed to
predict the skymaps of RMs and PI for a given GMF model. The robustness of the
inferred properties of the GMF is gauged by studying many combinations of
parametric field models and electron density models. We determine the pitch
angle of the local magnetic field (11+/-1 deg.), explore the evidence for a
grand-design spiral coherent magnetic field (inconclusive), determine the
strength of the toroidal and poloidal magnetic halo fields below and above the
disk (magnitudes the same for both hemispheres within 10
the half-height of the cosmic-ray diffusion volume (>2.9 kpc), investigate the
compatibility of RM- and PI-derived magnetic field strengths (compatible under
certain assumptions) and check if the toroidal halo field could be created by
the shear of the poloidal halo field due to the differential rotation of the
Galaxy (possibly). A set of eight models is identified to help quantify the
present uncertainties in the coherent GMF – spanning different functional
forms, data products and auxiliary input, and maximizing the differences in
their predictions. We present the corresponding skymaps of rates for
axion-photon conversion in the Galaxy, and deflections of ultra-high energy
cosmic rays.
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