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.
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