  The nonlinear propagation of a circularly polarized electromagnetic (CPEM)
wave in a strongly magnetized electron-positron-ion plasma is investigated. Two
coupled equations describing the interaction between a high-frequency CPEM wave
and the low-frequency electrostatic wake field are derived. It is found that
the generation of the wake fields depends on the presence of the ion species.
The wake field generation in turn leads to de-acceleration and frequency down
conversion of the electromagnetic pulse.
