Abstract
The well measured gas-phase abundances in
the low halo suggest that this region of the
Galaxy has total (gas plus dust) metal abundances
which are close to those in the solar
neighborhood. The gas-phase abundances in the halo
are generally higher than those seen in the disk,
however, this affect is likely due to the
destruction of dust in the halo clouds.
Observations of high velocity clouds (HVCs) in the
halo suggest that these clouds have metal
abundances which are substantially lower than
those measured for the local interstellar medium.
These determinations, however, are often of lower
quality than those for the low halo because of
uncertainties in the hydrogen abundances along the
sightlines, in the incorporation of elements into
dust, and in the partial ionization of the
clouds.
Spallation-dominated propagation of heavy cosmic rays and the
Local Interstellar Medium (LISM)
