Densities and filling factors of the DIG in the Solar neighbourhood
Abstract
For the first time we have combined dispersion measures and emission measures towards 38 pulsars at KNOWN distances from which we derived the mean electron density in clouds, Nc, and their volume filling factor, Fv, averaged along the line of sight. The emission measures were corrected for absorption by dust and contributions from beyond the pulsar distance. Results: The scale height of the electron layer for our sample is 0.93+/-0.13 kpc and the midplane electron density is 0.023+/-0.004 cm-3, in agreement with earlier results. The average density along the line of sight is <ne> = 0.018+/-0.002 cm-3 and nearly constant. Since <ne> = Fv Nc, an inverse relationship between Fv and Nc is expected. We find Fv(Nc) = (0.011+/-0.003) Nc-1.20+/-0.13, which holds for the ranges Nc = 0.05-1 cm-3 and Fv = 0.4-0.01. Near the Galactic plane the dependence of Fv on Nc is significantly stronger than away from the plane. Fv does not systematically change along or perpendicular to the Galactic plane, but the spread about the mean value of 0.08+/-0.02 is considerable. Conclusions: The inverse Fv-Nc relation is consistent with a hierarchical, fractal density distribution in the diffuse ionized gas (DIG) caused by turbulence. The observed near constancy of <ne> then is a signature of fractal structure in the ionized medium, which is most pronounced outside the thin disk.