  According to magnetohydrodynamics (MHD), the encounter of two collisional
magnetized plasmas at high velocity gives rise to shock waves. Investigations
conducted so far have found that the same conclusion still holds in the case of
collisionless plasmas. For the case of a flow-aligned field, MHD stipulates
that the field and the fluid are disconnected, so that the shock produced is
independent of the field. We present a violation of this MHD prediction when
considering the encounter of two cold pair plasmas along a flow-aligned
magnetic field. As the guiding magnetic field grows, isotropization is
progressively suppressed, resulting in a strong influence of the field on the
resulting structure. A micro-physics analysis allows to understand the
mechanisms at work. Particle-in-cell simulations also support our conclusions
and show that the results are not restricted to a strictly parallel field.
