A reexamination of the distribution of Galactic free electrons

We present a list of 109 pulsars with independent distance information comp iled from the literature. Since the compilation of Frail & Weisberg, there are 35 pulsars with new distance estimates and 25 pulsars for which the dis tance or distance uncertainty have been revised. We used this data to fit a smooth, axisymmetric, two-disk model of the distribution of Galactic elect rons. The two exponential model components have mean local midplane densiti es at the solar circle of 2.03 x 10(-2) and 0.71 x 10(-2) cm(-3), and scale heights of 1.07 and 0.053 kpc. The thick component shows very little radia l variation, while the second has a radial scale length of only a few kilop arsecs. We also examined a model that varies as sech(2) x, rather than exp (-x), in both the radial and vertical directions. We prefer this model with no midplane cusp but find that the Dt parameters essentially describe the same electron distribution. The distances predicted by this distribution ha ve a similar scatter to that produced by the more complex model of Taylor & Cordes. We examine the pulsars that deviate strongly from this model. Ther e are two regions of enhanced dispersion measure, one of which correlates w ell with the Sagittarius-Carina spiral arm. We find that the scatter of the observed dispersion measure from the model is not fitted well by either a normal or a lognormal distribution of lump sizes but may be caused instead by the uncertainties in the distances.