Bunching mechanism for coherent curvature radiation in pulsar magnetospheres

A relativistic plasma consisting of electron-positron pairs produced by photons initially has identical charge and current distributions in its two components. We show that when such a plasma moves along a curved field line of a rotating pulsar magnetosphere, a net charge density and a relative streaming between the electrons and the positrons must develop. The resulting two-stream instability bunches charges on a time scale ..gamma../sub plus-or-minus/ /sup 2/..omega../sup -1/ with ..gamma../sub plus-or-minus/ the average laboratory Lorentz factor and ..omega.. the laboratory plasma mode frequency. For an electron-positron plasma produced in a pulsar magnetosphere, the resulting charge bunching causes curvature radiation to be coherent in the microwave region. The growth rate of the instability approx.10/sup 3/-10/sup 4/ s/sup -1/ suffices to yield a large wave amplitude within the light cylinder. The results are generalized to oblique rotator geometries. For the unstable modes considered, inclusion of interaction with the radiation field can lead to amplication of bunching.