Time-dependent galactic winds
  Cosmic rays (CRs) are transported out of the galaxy by diffusion and
advection due to streaming along magnetic field lines and resonant scattering
off self-excited Magneto-Hydro-Dynamic (MHD) waves. Thus momentum is
transferred to the plasma via the frozen-in waves as a mediator assisting the
thermal pressure in driving a galactic wind. Galactic CRs (GCRs) are
accelerated by shock waves generated in supernova remnants (SNRs), and they
propagate from the disc into the halo. Therefore CR acceleration in the halo
strongly depends on the inner disc boundary conditions. We performed
hydrodynamical simulations of galactic winds in flux tube geometry appropriate
for disc galaxies, describing the CR diffusive-advective transport in a
hydrodynamical fashion (by taking appropriate moments of the Fokker-Planck
equation) along with the energy exchange with self-generated MHD waves. Our
time-dependent CR hydrodynamic simulations confirm that the evolution of
galactic winds with feedback depends on the structure of the galactic halo. In
case of a wind-structured halo, the wind breaks down after the last super nova
(SN) has exploded. The mechanism described here offers a natural and elegant
solution to explain the power-law distribution of CRs between the `knee' and
the `ankle'. The transition will be naturally smooth, because the Galactic CRs
accelerated at SN shocks will be `post-accelerated' by shocks generated at the
inner boundary and travelling through the halo.
