Electron Density Structure of the Local Galactic Disk

Pulsar dispersion measures (DMs) have been used to model the electron density of the interstellar medium (ISM) in the Galactic disk as a plane-parallel medium, despite significant scatter in the DM-distance distribution and strong evidence for inhomogeneities in the ISM. We use a sample of pulsars with independent distance measurements to evaluate a model of the local ISM in the thick disk of the Galaxy that incorporates turbulent fluctuations, clumps, and voids in the electron density. The latter two components are required because about one-third of the lines of sight are discrepant from a strictly plane-parallel model. A likelihood analysis for smooth components of the model yields a scale height $z_0={1.57}_{-0.14}^{+0.15}$ kpc and a mid-plane density n ₀ = 0.015 ± 0.001 cm⁻³. The scatter in the DM-distance distribution is dominated by clumps and voids but receives significant contributions from a broad spectrum of density fluctuations, such as a Kolmogorov spectrum. The model is used to identify lines of sight with outlier values of DM. Three of these pulsars, J1614−2230, J1623−0908, and J1643−1224, lie behind known H ii regions, and the electron density model is combined with Hα intensity data to constrain the filling factors and other substructure properties of the H ii regions (Sh 2–7 and Sh 2–27). Several pulsars also exhibit enhanced DM fluctuations that are likely caused by an intersection of their lines of sight with the superbubble GSH 238+00+09.