  We show here 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-alfvenic fastly growing mode which is the same that was found by Bell
(2004) by treating the plasma in the MHD approximation. The kinetic approach we
present is more powerful in that it allows us to investigate different models
for the compensation of the cosmic ray current in the background upstream
plasma.
