During the past 20 years, it has been established by several different means
that the ISM contains an important gaseous component with
temperature 
of 
to 
K (Spitzer 1990). Such gas is called coronal, by
analogy with the solar corona. Cooler interstellar gas is heated to
temperatures above this range by supernova explosions (Cox & Smith 1974). At these
temperatures the gas radiates soft X-rays and cools to about 
K,
where it may remain until it is heated again by another blast wave.
Depending on the mean density of the gas and the rate of energy input
by supernovae, a small (10%) to large (90%) fraction of the
ISM may
be filled with gas at of about 
to K (McKee & Ostriker 1977). Under
certain circumstances, a galactic fountain might exist, in which the
heated gas escapes into the galactic halo, where it could cool and
rain down on the disk (Shapiro & Field 1976).
The Sun is thought to be surrounded by a local bubble with a radius of 100 pc containing coronal gas that produces much, if not all, of the soft X-ray background radiation at energies below about 0.25 keV (McCammon & Sanders 1990). The fact that the background is brighter at the galactic poles, however, has led some investigators to suggest that at least some of the radiation is due to coronal gas in the galactic halo, the diffuse region that surrounds the main disk, at distances of several kiloparsecs from the galactic plane. Recent ROSAT (Roentgen Satellite) observations of shadows in the soft X-ray background that are attributable to foreground clouds tend to support this view (Marshall & Clark 1984; Burrows & Mendenhall 1991; Snowden et al. 1991). Observations with IUE have established that there is indeed ionized gas in the halo but have not established that the temperature of the halo gas is high enough to produce X-rays (Savage & Massa 1987).
Ultraviolet observations of interstellar OVI absorption
lines with Copernicus first established the existence of coronal gas in
the galactic disk, but could not determine whether such gas extends far
into the halo, because the telescope was unable to observe the faint objects
needed to probe the distant parts of the halo. Jenkins (1978) found that
the mean density of OVI ions in the disk is
and tentatively suggested a scale
height of only 300 pc to describe the distribution of this gas
away from the galactic plane. Such a small scale height would imply
that the hot gas is confined to a region fairly close to the galactic disk.
The OVI ion appears with significant fractional abundance only at 
K for gas in collisional equilibrium, and there are no
known sources of ionizing radiation that could explain widespread
OVI gas by means of photoionization. On the other hand, the lower ionization lines attributed to gas far above the
galactic plane, such
as CIV and SiIV, can be explained by photoionization
from a combination of halo stars and extragalactic UV background
radiation. Thus a crucial test of the nature of the galactic halo can
be made by searching for OVI absorption toward extragalactic
objects at high galactic latitudes. A hot corona will have a large
column density 
(total number of
particles per unit area integrated along the line of sight) of
OVI (Edgar & Chevalier 1986), whereas a cooler, photoionized halo, supported at a
large distance from the galactic plane by something other than its
thermal pressure, will have a much smaller column density of this ion
(Fransson & Chevalier 1985).
On the Astro-1 mission, HUT was used to observe the far-ultraviolet spectrum of the quasar
3C273 (Davidsen et al. 1993). At galactic latitude 
, this line of sight passes almost vertically through the
entire halo,
providing an excellent probe for coronal gas. The quasar is a good background source for such an observation because it is expected
to have an intrinsically smooth spectrum, with no features at
the rest wavelength where OVI absorption in our Galaxy
would occur. The HUT spectrum of 3C273 has strong absorption features
at 1032 and 1038 Å, the wavelengths of the OVI
doublet (see Figure 
). However, there are several
interstellar absorption lines that are expected to contribute to the
feature at 1038 Å based on absorption lines seen with HST at
longer wavelengths. There are also possible contributions to the
OVI features from intergalactic hydrogen clouds whose
Lyman-
absorption has been seen with HST. The clouds discovered
in the Virgo cluster have a redshift that causes their Lyman-
lines to contribute to the 1032 Å feature, while Lyman-
from
a higher redshift cloud may also contribute to the 1038 Å feature. It is
possible to place strong upper limits on all of these contaminating
features, however, and the result is that OVI absorption is
also required to produce the strong features seen with HUT.
Davidsen et al. (1993) find (OVI) is very likely 
. When compared with the mean
density in the disk derived from Copernicus observations (Jenkins 1978),
this suggests a scale height of at least 3 kpc, clearly placing
the hot
gas in the galactic halo if this line of sight is not atypical. A
model for a photoionized halo predicted (OVI) 
(Fransson & Chevalier 1985), whereas a model based on a radiatively
cooling corona or galactic fountain predicted (OVI)
(Edgar & Chevalier 1986). Clearly, the former model is
excluded by the HUT result, but the latter model is supported. However, the radiative cooling model of Edgar & Chevalier
(1986) does not actually predict enough of the lower ionization species
SiIV and CIV to match the observations of 3C273,
indicating that the situation is more complicated. Recent
work (Shapiro & Benjamin 1991) on the
galactic fountain has shown that self-photoionization of the
radiatively cooling gas alters the relative abundances of the lower
ionization states appreciably. When this effect is incorporated in
the calculations, it is found that the predicted column density ratios
of OVI, NV, CIV and SiIV
come into close agreement with the observations. Therefore, HUT has
provided new evidence supporting the existence of a hot galactic
corona, with K, probably originating in a galactic
fountain. Of course, it is still necessary to measure the OVI column density along several other lines of sight to prove that the hot
gas is widespread in the halo and to map out its distribution. These
observations will be attempted with HUT on the Astro-2 mission.
