Interaction of Cosmic Rays with Cold Clouds in Galactic Halos
  We investigate the effects of cosmic ray (CR) dynamics on cold, dense clouds
embedded in a hot, tenuous galactic halo. If the magnetic field does not
increase too much inside the cloud, the local reduction in Alfv\'en speed
imposes a bottleneck on CRs streaming out from the star-forming galactic disk.
The bottleneck flattens the upstream CR gradient in the hot gas, implying that
multi-phase structure could have global effects on CR driven winds. A large CR
pressure gradient can also develop on the outward-facing edge of the cloud.
This pressure gradient has two independent effects. The CRs push the cloud
upward, imparting it with momentum. On smaller scales, the CRs pressurize cold
gas in the fronts, reducing its density, consistent with the low densities of
cold gas inferred in recent COS observations of local $L_{*}$ galaxies. They
also heat the material at the cloud edge, broadening the cloud-halo interface
and causing an observable change in interface ionic abundances. Due to the much
weaker temperature dependence of cosmic ray heating relative to thermal
conductive heating, CR mediated fronts have a higher ratio of low to high ions
compared to conduction fronts, in better agreement with observations. We
investigate these effects separately using 1D simulations and analytic
techniques.
