Global distribution of far-ultraviolet emissions from highly ionized gas
  in the Milky Way
  We present all-sky maps of two major FUV cooling lines, C IV and O VI, of
highly ionized gas to investigate the nature of the transition-temperature gas.
From the extinction-corrected line intensities of C IV and O VI, we calculated
the gas temperature and the emission measure of the transition-temperature gas
assuming isothermal plasma in the collisional ionization equilibrium. The gas
temperature was found to be more or less uniform throughout the Galaxy with a
value of (1.89 $\pm$ 0.06) $\times$ $10^5$ K. The emission measure of the
transition-temperature gas is described well by a disk-like model in which the
scale height of the electron density is $z_0=6_{-2}^{+3}$ kpc. The total mass
of the transition-temperature gas is estimated to be approximately
$6.4_{-2.8}^{+5.2}\times10^9 M_{\bigodot}$. We also calculated the
volume-filling fraction of the transition-temperature gas, which was estimated
to be $f=0.26\pm0.09$, and varies from $f\sim0.37$ in the inner Galaxy to
$f\sim0.18$ in the outer Galaxy. The spatial distribution of C IV and O VI
cannot be explained by a simple supernova remnant model or a three-phase model.
The combined effects of supernova remnants and turbulent mixing layers can
explain the intensity ratio of C IV and O VI. Thermal conduction front models
and high-velocity cloud models are also consistent with our observation.
