  Efficient acceleration of cosmic rays (via the mechanism of diffusive shock
acceleration) requires turbulent, amplified magnetic fields in the shock's
upstream region. We present results of multidimensional particle-in-cell
simulations aimed at observing the magnetic field amplification that is
expected to arise from the cosmic-ray current ahead of the shock, and the
impact on the properties of the upstream interstellar medium. We find that the
initial structure and peak strength of the amplified field is somewhat
sensitive to the choice of parameters, but that the field growth saturates in a
similar manner in all cases: the back-reaction on the cosmic rays leads to
modification of their rest-frame distribution and also a net transfer of
momentum to the interstellar medium, substantially weakening their relative
drift while also implying the development of a modified shock. The upstream
medium becomes turbulent, with significant spatial fluctuations in density and
velocity, the latter in particular leading to moderate upstream heating; such
fluctuations will also have a strong influence on the shock structure.
