Connecting Synchrotron, Cosmic Rays, and Magnetic Fields in the Plane of
  the Galaxy
  We extend previous work modeling the Galactic magnetic field in the plane
using synchrotron emission in total and polarised intensity. In this work, we
include a more realistic treatment of the cosmic-ray electrons using the
GALPROP propagation code optimized to match the existing high-energy data. This
addition reduces the degeneracies in our previous analysis and when combined
with an additional observed synchrotron frequency allows us to study the
low-energy end of the cosmic-ray electron spectrum in a way that has not
previously been done. For a pure diffusion propagation, we find a low-energy
injection spectrum slightly harder than generally assumed; for J(E) \propto
E^{\alpha}, we find {\alpha} = -1.34 \pm 0.12, implying a very sharp break with
the spectrum above a few GeV. This then predicts a synchrotron brightness
temperature spectral index, {\beta}, on the Galactic plane that is -2.8 <
{\beta} < -2.74 below a few GHz and -2.98 < {\beta} < -2.91 up to 23 GHz. We
find that models including cosmic-ray re-acceleration processes appear to be
incompatible with the synchrotron data.
