Cosmic ray-driven galactic winds: transport modes of cosmic rays and
  Alfv\'en-wave dark regions
  Feedback mediated by cosmic rays (CRs) is an important process in galaxy
formation. Because CRs are long-lived and because they are transported along
magnetic field lines independently of any gas flow, they can efficiently
distribute their feedback energy within the galaxy. We present an in-depth
investigation of (i) how CRs launch galactic winds from a disc that is forming
in a $10^{11} \mathrm{M}_\odot$ halo and (ii) how CR transport affects the
dynamics in a galactic outflow. To this end, we use the Arepo moving-mesh code
and model CR transport with the two-moment description of CR hydrodynamics.
This model includes the CR interaction with gyroresonant Alfv\'en waves that
enables us to self-consistently calculate the CR diffusion coefficient and CR
transport speeds based on coarse-grained models for plasma physical effects.
This delivers insight into key questions such as whether the effective CR
transport is streaming-like or diffusive-like, how the CR diffusion coefficient
and transport speed change inside the circumgalactic medium (CGM), and to what
degree the two-moment approximation is needed to faithfully capture these
effects. We find that the CR-diffusion coefficient reaches a steady-state in
most environments with the notable exception of our newly discovered
Alfv\'en-wave dark regions where the toroidal wind magnetic field is nearly
perpendicular to the CR pressure gradient so that CRs are unable to excite
gyroresonant Alfv\'en waves. However, CR transport itself cannot reach a
steady-state and is not well described by either the CR streaming paradigm, the
CR diffusion paradigm or a combination of both.
