  Cosmic rays (CRs) and magnetic fields are dynamically important components in
the Galaxy, and their energy densities are comparable to that of the turbulent
interstellar gas. The interaction of CRs and Galactic magnetic fields produces
synchrotron radiation clearly visible in the radio regime. Detailed
measurements of synchrotron radiation averaged over the line-of-sight (LOS),
so-called synchrotron emissivities, can be used as a tracer of the CR density
and Galactic magnetic field (GMF) strength. Our aim is to model the synchrotron
emissivity in the Milky Way using a 3 dimensional dataset instead of
LOS-integrated intensity maps on the sky. Using absorbed HII regions we can
measure the synchrotron emissivity over a part of the LOS through the Galaxy,
changing from a 2 dimensional to a 3 dimensional view. Performing these
measurements on a large scale is one of the new applications of the window
opened by current low frequency arrays. Using various simple axisymmetric
emissivity models and a number of GMF-based emissivity models we can simulate
the synchrotron emissivities and compare them to the observed values in the
catalog. We present a catalog of low-frequency absorption measurements of HII
regions, their distances and electron temperatures, compiled from literature.
These data show that the axisymmetric emissivity models are not complex enough,
but the GMF-based emissivity models deliver a reasonable fit. These models
suggest that the fit can be improved by either an enhanced synchrotron
emissivity in the outer reaches of the Milky Way, or an emissivity drop near
the Galactic center. State-of-the-art GMF models plus a constant CR density
model cannot explain low-frequency absorption measurements, but the fits
improved with slight (ad-hoc) adaptations. It is clear that more detailed
models are needed, but the current results are very promising.
