We have studied the possibility of exciting a wake field in an electron–positron plasma by the injection of relativistic electron bunches. We have investigated the change in frequency of electronmagnetic radiation propagating through the resulting inhomogeneity. Our main conclusion is that it is not necessary to invoke a magnetic field in order to excite a wake field in an electron–positron plasma. As shown here, the necessary charge separation could come from the injection of energetic particle bunches. This mechanism of wake-field creation and subsequent particle/photon acceleration could operate in a pulsar magnetosphere, where particle bunches are extracted energetically from the pulsar surface. For example, pulsar radio emissions exhibit ultra-short intensity variations within individual pulses with time scales ranging from 1 μs to 1 ms (Cordes 1979). Several authors have proposed (Chian & Kennel 1983; Mofiz, De Angelis & Forlani 1985; Mikhailovskii, Onishchenko & Tatarinov 1985) that these pulsations can be explained as being due to soliton formation in the pulsar magnetosphere.
We believe that these micropulsations can also be explained by modulation of the radiation caused by the wake field. In order to have a quantitative estimation, we need to take into account the effects of thermal motion of particles as well as plasma inhomogeneity, etc. The present results should also be useful for understanding nonolinear photon motion in cosmic plasmas, such as those found in the early universe and in active galactic nuclei (Tajima & Taniuti 1990).