THE SCALE HEIGHT OF THE GALACTIC FREE-ELECTRON CLOUD

Accurate distance measurements toward sixty-one pulsars in and around the disk of the Galaxy show a mean electron density for the interstellar medium of [n(e)] congruent-to 0.033 +/- 0.002 cm-3. The additional discovery of 29 pulsars in globular clusters at high galactic latitudes indicates an electron scale height of between 0.53 and 0.84 kpc. The number of photons available for ionization of the ISM is calculated and compared with the number of photons that theory predicts are required for the degree of ionization observed. It is found that UV photons from OB stars, supernovae, and planetary nebulae are sufficient to produce the necessary power, but only if the fraction of photons that are used to maintain the ionized gas approaches 40%, and only if the majority of the electron gas is found at a temperature of approximately 10(4) K.