The column density and scale height of free electrons in the galactic disk

The recent discoveries of millisecond pulsars in globular clusters located far from the Galactic plane provide the first direct measurements of the total extent and column density of diffuse ionized gas within the Galactic disk. The dispersion measures of the pulsars in the clusters, 47 Tuc at z equals about -3.3 kpc and M15 at z equals about -4.3 kpc imply a column density of (0.8-1.4) x to the 20th/sq cm between the clusters and the midplane. When combined with the dispersion measures of pulsars located closer to the Galactic plane, this result indicates a free electron scale height H(e) = 1500 pc. Diffuse H(+) thus accounts for about one-quarter of the total column density of interstellar atomic hydrogen near the solar circle and appears to be the dominant state of the interstellar gas at an absolute value of z greater than 1000 pc. 46 refs