17 Location of emitting regions

Summary

Apart from the thermal radiation observable in X-rays from the surface of a neutron star, the sources of electromagnetic pulsar emissions are located in two distinct regions of the magnetosphere: the polar cap and the outer gap. These are two regions which are defined by the simple large-scale geometry of the magnetosphere outlined in Chapter 2 (although some further geometry is needed to extend the model for large angles between the rotation and magnetic axes; see Kapoor & Shukre 1998).

Outside these two regions the large electric field induced by the rotating magnetic dipole is shielded by the ambient charged plasma. The outer gap is a surface located adjacent to the open magnetic field lines, and extending to and beyond the light cylinder; electrons and positrons accelerated within this gap generate gamma rays at the highest observable energies. The mechanism for coherent radio emission is not understood, but the location can be deduced from the rich and sometimes complex integrated pulse profiles described in Chapter 15; most radio emission from normal pulsars originates in the polar cap, while some, especially from young pulsars, also originates in the outer gap.

Relativistic aberration has a major effect on the outer gap radiation, which occurs at radii approaching the velocity-of-light cylinder; it has almost no observable effect on polar cap emission, which occurs comparatively close to the star.