Energetic particle acceleration in collisionless shocks results in extended cosmic ray current in the shock upstream. We found a cosmic ray current driven instability of long-wavelength fluctuations in the shock upstream (the wavelengths are longer than the cosmic ray mean free path). The growth rates for the unstable long-wavelength fluctuations are calculated using an appropriate averaging method for the hybrid kinetic/MHD equations of a multicomponent turbulent plasma with anisotropic distribution of relativistic particles. The growth rates are compared with that of the non-resonant short-wavelength instability investigated by Bell (2004). The amplification of the long-wavelength magnetic field fluctuations is essential for the problem of maximal energy of the particles accelerated by Fermi mechanism in collisionless shocks of SNRs.

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