The Nature of Ionized Gas in the Milky Way Galactic Fountain
  We address the spatial scale, ionization structure, mass and metal content of
gas at the Milky Way disk-halo interface detected as absorption in the
foreground of seven closely-spaced, high-latitude halo blue horizontal branch
stars (BHBs) with heights z = 3 - 14 kpc. We detect transitions that trace
multiple ionization states (e.g. CaII, FeII, SiIV, CIV) with column densities
that remain constant with height from the disk, indicating that the gas most
likely lies within z < 3.4 kpc. The intermediate ionization state gas traced by
CIV and SiIV is strongly correlated over the full range of transverse
separations probed by our sightlines, indicating large, coherent structures
greater than 1 kpc in size. The low ionization state material traced by CaII
and FeII does not exhibit a correlation with either N$_{\rm HI}$ or transverse
separation, implying cloudlets or clumpiness on scales less than 10 pc. We find
that the observed ratio log(N_SiIV/ N_CIV), with a median value of
-0.69+/-0.04, is sensitive to the total carbon content of the ionized gas under
the assumption of either photoionization or collisional ionization. The only
self-consistent solution for photoionized gas requires that Si be depleted onto
dust by 0.35 dex relative to the solar Si/C ratio, similar to the level of Si
depletion in DLAs and in the Milky Way ISM. The allowed range of values for the
areal mass infall rate of warm, ionized gas at the disk-halo interface is
0.0003 < dM_gas / dtdA [M_sun kpc^-2 yr^-] < 0.006. Our data support a physical
scenario in which the Milky Way is fed by complex, multiphase processes at its
disk-halo interface that involve kpc-scale ionized envelopes or streams
containing pc-scale, cool clumps.
