A more detailed look at Galactic magnetic field models: using free-free
  absorption in HII regions
  The observed interaction product of Cosmic Rays (CRs) and Galactic magnetic
fields (GMF) is the Galactic synchrotron emission integrated over the
line-of-sight (LOS). The GMF strength and morphology and the CR density can be
probed by comparing this tracer to simulations using existing GMF models and CR
density models. Our aim is to provide insight into these parameters by
exploring and explaining the differences between simulations and observations
of synchrotron intensity. At low radio frequencies HII regions become opaque
due to free-free absorption. Using these HII regions we can measure the
synchrotron intensity over a part of the LOS through the Galaxy. The measured
intensity per unit path length, i.e. the emissitivity, for HII regions at
different distances, will allow probing variation in synchrotron emission in
the third dimension of distance. Using a number of existing GMF models in
conjunction with the Galactic CR modeling code GALPROP we can simulate these
synchrotron emissivities. We present an updated catalog of low-frequency
absorption measurements of HII regions compiled from the literature. We report
a simulated emissivity that shows a compatible trend for HII regions that are
near to the observer. And we observe a systematically increasing synchrotron
emissivity for HII regions that are far from the observer, which is not
compatible with simulated values. Current GMF and CR density models cannot
explain low-frequency absorption measurements. One possibility is that
distances to all HII regions at the kinematic 'far' distance are wrong, though
this is unlikely as it ignores all evidence in the literature. However, a
detection bias due to the nature of this tracer requires us to keep in mind
that certain sources may be missed in an observation. The other possibilities
are an enhanced emissivity in the outer Galaxy or a diminished emissivity in
the inner Galaxy.
