1994A&A...289..539S

We analyze high-resolution Na I and Ca II interstellar absorption line data obtained in an earlier spectroscopic survey of 57 stars along extended sight lines through the Galactic disk and halo. We find that the Na I lines trace a diffuse cloudy medium and the Ca II lines trace both the cloudy medium and a more extended (intercloud) medium. High latitude and interarm sight lines that do not cross spiral arms have clouds that are more diffuse on average than those along sight lines that cross spiral arms. Spiral structure may play an important role in determining the average absorption properties along extended sight lines and/or interesting physical differences may exist between sight lines that cross spiral arms and those that do not. These might include a harder radiation field and/or higher electron temperatures along the high latitude and "clean" interarm sight lines. On average, 10% of the Ca II column density occurs at velocities forbidden by the Galactic rotation law by more than 10km/s. In contrast, only a small percentage of the Na I column density occurs at these velocities. The Ca II to Na I ratio increases by a factor of 15 over forbidden velocities from 0 to 50 km/s and rises rapidly thereafter. A two component model of the Ca II column density per unit velocity over the range l=325deg to 360deg indicates that two distinct distributions exist, one with σ=8km/s and one with σ=21km/s. As much as 60% of the Ca II column density at forbidden velocities may be associated with the faster distribution, which we attribute to warm intercloud material. We estimate exponential scale heights of 0.4-0.5kpc for the neutral gas traced by the E(B-V), Na I, and H I distributions along the low density sight lines, and we find that Ca II has a larger scale height of 0.8kpc.