  High-quality Far-Ultraviolet Spectroscopic Explorer (FUSE) observations at 20
km /s resolution of interstellar and intergalactic absorption from 910 to 1187
A are presented for the X-ray bright BL Lac object Mrk 421. In this study we
consider the O VI absorption between -140 to 165 km /s and its relationship to
the lower ionization absorption and to the strong absorption produced by O VII
and O VIII at X-ray wavelengths. The O VI absorption extending from -140 to 60
km /s is associated with strong low ionization gas absorption and originates in
the Galactic thick disk / halo. This O VI appears to be produced by a
combination of processes, including conductive interfaces between warm and hot
gas and possibly cooling Galactic Fountain gas and hot halo gas bubbles. The O
VI absorption extending from 60 to 165 km /s has unusual ionization properties
in that there is very little associated low ionization absorption, with the
exception of C III. This absorption is not observed toward two foreground halo
stars, implying that it occurs in gas more distant than 3.5 kpc from the
Galactic disk. Over the 60 to 165 km/s velocity range, O VI and C III
absorption have the same kinematic behavior. N(O VI)/N(C III) = 10+/-3 over the
60 to 120 km/s velocity range. Given the association of O VI with C III, it is
unlikely that the high velocity O VI co-exists with the hotter gas responsible
for the O VII and O VIII absorption. The O VI positive velocity absorption wing
might be tracing cooler gas entrained in a hot Galactic Fountain outflow. The O
VII and O VIII absorption observed by Chandra and XMM-Newton may trace the hot
gas in a highly extended (~100 kpc) Galactic corona or hot gas in the Local
Group.
