19 The emission mechanisms

Summary

More than 40 years after the discovery of pulsars, and despite many attempts to assemble and analyse the very detailed observational data, it is still not possible to give a clear exposition of the processes by which pulsars emit beams of radio waves. The later discovery of intense gamma-ray emission has proved more amenable to analysis, and the observed characteristics can be related to the particle energies and the geometry of the magnetosphere. In both radio and gamma emissions, the source of energy is the enormous electric field which is induced by the rapid rotation of the highly magnetised neutron star. This electric field accelerates electrons and positrons to high relativistic energies; this occurs in two emission regions, involving respectively the open field lines close to the polar cap, and the outer gap. The high-energy photons emitted from the outer gap are curvature or synchrotron radiation from these primary relativistic particles; in contrast, the radio emission from both regions is coherent emission from a plasma created by the primary particle stream.

The two locations

The rapid rotation and strong dipole field of a typical pulsar give a maximum available potential of order 10¹⁴–10¹⁵ volts. In a fully developed, co-rotating and static magnetosphere this would be shielded by the electrostatic field of the charged particle density n_GJ, as envisaged by Goldreich and Julian.