Solenoidal Improvements for the JF12 Galactic Magnetic Field Model
  The popular JF12 analytic model by Jansson & Farrar provides a quantitative
description of the Galaxy's large-scale magnetic field, which is widely used in
various astrophysical applications. However, both the poloidal X-type component
and the spiral disk component of JF12 exhibit regions in which the magnetic
divergence constraint is violated. We first propose a cure for this problem,
resulting in a truly solenoidal large-scale spiral field. Second, the otherwise
straight field lines of the X-type component exhibit kinks in the Galactic
plane that, in addition to implying the presence of a singular current sheet,
may pose difficulties for e.g., numerical tracing of cosmic-ray particles. We
propose and discuss two possible strategies to mitigate this problem. Although
all corrections are kept as minimal as possible, the extended set of model
parameters will have to be carefully readjusted in order to fully restore the
agreement to observational data that the unmodified JF12 field is based on.
Furthermore, the performance of our improved version of the field model is
quantitatively assessed by test simulations using the CRPropa Galactic
cosmic-ray propagation code.
