  We present the results from a particle-in-cell (PIC) simulation that models
the interaction between a spatially localized electron-positron cloud and an
electron-ion plasma. The latter is permeated by a magnetic field that is
initially spatially uniform and aligned with the mean velocity vector of the
pair cloud. The pair cloud expels the magnetic field and piles it up into an
electromagnetic piston. Its electromagnetic field is strong enough to separate
the pair cloud from the ambient plasma in the direction that is perpendicular
to the cloud propagation direction. The piston propagates away from the spine
of the injected pair cloud and it accelerates the protons to a high
nonrelativistic speed. The accelerated protons form an outer cocoon that will
eventually become separated from the unperturbed ambient plasma by a fast
magnetosonic shock. No electromagnetic piston forms at the front of the cloud
and a shock is mediated here by the filamentation instability. The final plasma
distribution resembles that of a hydrodynamic jet. Collisionless plasma jets
may form in the coronal plasma of accreting black holes and the interaction
between the strong magnetic field of the piston and the hot pair cloud may
contribute to radio emissions by such objects.
