Introduction

The neutral interstellar gas in the Milky Way is largely confined to the galactic plane with a density stratification away from the plane that is approximately given by n(HI)≈n(HI)0e-|z|/h, with n(HI)0≈1.2 atoms cm-3 and h≈0.12 kpc. However, this simple exponential distribution does not adequately describe a very extended and highly ionized component of the interstellar gas that is commonly referred to as galactic halo gas or galactic corona gas. The density stratification of the halo gas is very uncertain but may have a scale height that exceeds by a factor of 30 that of the neutral disk gas.

The observational study of galactic halo gas is a very young field – the youngest of those subjects included in this volume. That a hot (104–106 K) and extended (z ≈10–30 kpc) gaseous halo surrounds the Milky Way was suggested by Shklovsky (1952) based on measurements of non-thermal radio emission from the galaxy and by Spitzer (1956) based on the apparent stability of high-latitude interstellar clouds. Spitzer noted that stars at z distances exceeding 0.5 kpc more frequently exhibit high velocity interstellar Ca II optical absorption lines than do stars at smaller z. He concluded that an appreciable fraction of the high-velocity clouds producing these absorption features must exist more than 0.5 kpc from the plane. The basic problem associated with a cloud at z > 0.5 kpc is its instability to outward expansion unless the cloud is in near pressure equilibrium with an external medium. The external medium was postulated to be a high-temperature low-density gas – the galactic corona.