Cosmic Ray Acceleration of Cool Clouds in the Circumgalactic Medium
  We investigate a mechanism for accelerating cool (10$^4$ K) clouds in the
circumgalactic medium (CGM) with cosmic rays (CRs), possibly explaining some
characteristics of observed high velocity clouds (HVCs). Enforcing CRs to
stream down their pressure gradient into a region of slow streaming speed
results in significant buildup of CR pressure which can accelerate the CGM. We
present the results of the first two-dimensional magnetohydrodynamic (MHD)
simulations of such `CR bottlenecks,' expanding on simpler simulations in 1D
from \cite{wiener17a}. Although much more investigation is required, we find
two main results. First, radiative cooling in the interfaces of these clouds is
sufficient to keep the cloud intact to CR wave heating. Second, cloud
acceleration depends almost linearly with the injected CR flux at low values
(comparable to that expected from a Milky Way-like star formation rate), but
scales sublinearly at higher CR fluxes in 1D simulations. 2D simulations show
hints of sublinear dependence at high CR fluxes but are consistent with pure
linear dependence up to the CR fluxes tested. It is therefore plausible to
accelerate cool clouds in the CGM to speeds of hundreds of km s$^{-1}$.
