  We have recently identified several high velocity (V < -100 km/s) clouds in
the directions of Mrk 509 and PKS 2155-304 that have unusual ionization
properties. The clouds exhibit strong C IV absorption with little or no
detectable low ion (C II, Si II) absorption or H I 21cm emission. As the
closest known analog to the outer diffuse halos of damped Ly-alpha absorbers
and the low H I column density metal line absorption systems seen in the
spectra of high redshift quasars, these "C IV-HVCs" present unique
opportunities for relating the conditions within the Milky Way halo and nearby
intergalactic gas to the properties of galactic halos at higher redshift.
  The C IV-HVCs have ionization properties consistent with photoionization by
extragalactic background radiation, though some contribution by collisional
ionization within a hot plasma cannot be ruled out. The clouds are probably low
density [n(H) ~ 10^-4 cm^-3], large [greater than several kiloparsecs], and
mostly ionized [n(HI)/n(H) ~ 10^-3] regions located well beyond the neutral gas
layer of the Galaxy. The presence of weak H I-HVCs detected through 21cm
emission near both sight lines indicates that the C IV-HVCs trace the extended,
ionized, low density regions of the H I-HVCs. Several lines of evidence,
including very low thermal pressures (P/k ~ 2 cm^-3 K), favor a location for
the C IV-HVCs in the Local Group or very distant Galactic halo.
