Modelling the Galactic distribution of free electrons
  In this paper we test 8 models of the free electron distribution in the Milky
Way that have been published previously, and we introduce 4 additional models
that explore the parameter space of possible models further. These new models
consist of a simple exponential thick disk model, and updated versions of the
models by Taylor & Cordes and Cordes & Lazio with more extended thick disks.
The final model we introduce uses the observed H-alpha intensity as a proxy for
the total electron column density, also known as the dispersion measure (DM).
We use the latest available data sets of pulsars with accurate distances
(through parallax measurements or association with globular clusters) to
optimise the parameters in these models. In the process of fitting a new scale
height for the thick disk in the model by Cordes & Lazio we discuss why this
thick disk cannot be replaced by the thick disk that Gaensler et al. advocated
in a recent paper. In the second part of our paper we test how well the
different models can predict the DMs of these pulsars at known distances.
Almost all models perform well, in that they predict DMs within a factor of
1.5-2 of the observed DMs for about 75% of the lines of sight. This is somewhat
surprising since the models we tested range from very simple models that only
contain a single exponential thick disk to very complex models like the model
by Cordes & Lazio. We show that the model by Taylor & Cordes that we updated
with a more extended thick disk consistently performs better than the other
models we tested. Finally, we analyse which sightlines have DMs that prove
difficult to predict by most models, which indicates the presence of local
features in the ISM between us and the pulsar. (abridged)
