  We show that a purely kinetic approach to the excitation of waves by cosmic
rays in the vicinity of a shock front leads to predict the appearance of a
non-alfv\'enic fastly growing mode which has the same dispersion relation as
that previously found by \cite{bell04} by treating the plasma in the MHD
approximation. The kinetic approach allows us to investigate the dependence of
the dispersion relation of these waves on the microphysics of the current which
compensates the cosmic ray flow. We also show that a resonant and a
non-resonant mode may appear at the same time and one of the two may become
dominant on the other depending on the conditions in the acceleration region.
We discuss the role of the unstable modes for magnetic field amplification and
particle acceleration in supernova remnants at different stages of the remnant
evolution.
