  Far-ultraviolet spectra of 11 AGNs observed by FUSE are analyzed to obtain
measures of O VI 1031.93 A absorption occurring over very long paths through
Milky Way halo gas. Strong O VI absorption is detected along 10 of 11 sight
lines. Values of log [N(O VI) sin|b|] range from 13.80 to 14.64 with a median
value of 14.21. The observations reveal the existence of a widespread but
irregular distribution of O VI in the Milky Way halo. Combined with estimates
of the O VI mid-plane density, n0 = 2x10^(-8) cm^(-3), from the Copernicus
satellite, the FUSE observations imply an O VI exponential scale height of
2.7+/-0.4 kpc. We find that N(C IV)/N(O VI) ranges from 0.15 in the disk to 0.6
along four extragalactic sight lines. The changing ionization state of the gas
from the disk to the halo is consistent with a systematic decrease in the scale
heights of Si IV, C IV, N V, to O VI from ~5.1 to ~2.7 kpc. While conductive
heating models can account for the highly ionized atoms at low |z|, a
combination of models (and processes) appears to be required to explain the
highly ionized atoms found in the halo. The greater scale heights of Si IV and
C IV compared to O VI suggest that some of the Si IV and C IV in the halo is
produced in turbulent mixing layers or by photoionization by hot halo stars or
the extragalactic background.
