  A collisionless shock wave is treated as a streaming plasma instability in
the interstellar medium (ISM). We assume that in a steady state, this
instability propagates through the ISM as a self-driven plasma instability,
whereby the parameters of the instability are determined such that it causes
density and velocity jumps as well as isotropization of the particle
velocities, which then must be in accordance with MHD theory. The process of
resonant interaction and particle scattering off such instability and their
backscattering to an upstream region is simulated. We find that some ISM
particles bounce off a shock and thus become suprathermal and eligible to enter
the process of acceleration to cosmic ray energies by the mechanism of
diffusive shock acceleration.
