Coherent emission mechanisms may be classified as (i) maser mechanisms, attributed to negative absorption by resonant particles in a resistive instability, (ii) a reactive or hydrodynamic instability, or (iii) to emission by bunches. Known coherent emission mechanisms in radio astronomy are plasma emission in solar radio bursts, maser emission in OH and other molecular line sources, electron-cyclotron maser emission from the planets, and pulsar emission. Pulsar radio emission is the brightest of all known coherent emission, and its brightness temperature is close to the maximum conceivable in terms of energy efficiency. Three possible pulsar radio emission mechanisms warrant serious consideration in polar cap models; here these are called coherent curvature emission, relativistic plasma emission, and free electron maser emission, respectively.

1. 1. Coherent curvature emission is attributed to emission by bunches. There is a fundamental weakness in existing theoretical treatments which do not allow for any velocity dispersion of the particles. There is no satisfactory mechanism for the formation of the required bunches, and were such bunches to form they would quickly lose their ability to emit coherently due to the curvature of the field lines.

2. 2. Relativistic plasma emission is a multistage emission process involving the generation of plasma turbulence and the partial conversion of this turbulence into escaping radiation. In pulsars the dispersion characteristics of the relativistic electron-positron plasma determines the form of the turbulence, which may be in either longitudinal waves or Alfvèn-like waves. Various instabilities have been suggested to produce turbulence, and a streaming instability is one possibility. Alternatively, in a detailed model proposed by Beskin et al. (1988) the instability depends intrinsically on the curvature of the field lines, and in a theory discussed by Kazbegi et al. (1988), a cyclotron instability generates the turbulence relatively far from the neutron star.

3. 3. Free electron maser emission or linear acceleration emission requires an oscillating electric field, postulated to be due to a large amplitude electrostatic wave. A recent analysis of this mechanism (Rowe 1992) shows that it allows emission in two different regimes that provide a possible basis for the interpretation of core and conal emission in pulsars. Effective maser emission seems to require Lorentz factors smaller than other constraints allow.

Other suggested theories for the emission mechanism include one that arises from a loophole in the proof that curvature absorption cannot be negative, and another that involves a closed “electrosphere” in which the radio emission is attributed to emission by bunches formed as a result of pair production due to a primary charge accelerated towards the star by its Coulomb field.