COHERENT RADIO-EMISSION FROM PULSARS

The high brightness temperature of pulsar radiation requires that the emission process be coherent. There are three possibilities in principle: emission by bunches; reactive instability, due to an intrinsically growing wave mode; and kinetic instability, which is maser action. The emission may be direct or indirect, depending on whether the radiation can escape to infinity through the pulsar magnetosphere, or must first be converted into another wave mode. Early models favoured either direct curvature emission by bunches or indirect emission due to a reactive beam instability, but before about 1980 it was realized that there are serious problems with both mechanisms. There are strong physical arguments against emission by bunches being viable, and the first detailed analysis suggested that the seemingly plausible alternative of maser curvature emission is impossible. Also the growth rates for beam instabilities were found too small to allow waves to grow effectively. Alternative emission mechanisms, including cyclotron and linear acceleration emissions, and variants on the existing mechanisms have been considered. In this paper the suggested emission mechanisms are reviewed from a plasma-physical viewpoint, and they are then compared to see how they might fit into a phenomenological model for pulsar radio emission.