  As the next step toward an improved large scale Galactic magnetic field
model, we present a simple comparison of polarised synchrotron and thermal dust
emission on the Galactic plane. We find that the field configuration in our
previous model that reproduces the polarised synchrotron is not compatible with
the WMAP 94 GHz polarised emission data. In particular, the high degree of dust
polarisation in the outer Galaxy (90deg < l < 270deg) implies that the fields
in the dust-emitting regions are more ordered than the average of
synchrotron-emitting regions. This new dust information allows us to constrain
the spatial mixing of the coherent and random magnetic field components in the
outer Galaxy. The inner Galaxy differs in polarisation degree and apparently
requires a more complicated scenario than our current model. In the scenario
that each interstellar component (including fields and now dust) follows a
spiral arm modulation, as observed in external galaxies, the changing degree of
ordering of the fields in dust-emitting regions may imply that the dust arms
and the field component arms are shifted as a varying function of
Galacto-centric radius. We discuss the implications for how the spiral arm
compression affects the various components of the magnetised interstellar
medium but conclude that improved data such as that expected from the Planck
satellite will be required for a thorough analysis.
